S
ome scan tools call it the global OBD II mode, while others describe it as the OBD II generic mode. The OBD II generic mode allows a technician to attach his scan tool to an OBD II-compliant vehi-cle and begin collecting data without entering any VIN information into the scan tool. You may need to specifically select “OBD II Generic” from the scan tool menu. Some scan tools may need a software module or personality key be-fore they’ll work in generic OBD II test mode.The original list of generic data pa-rameters mandated by OBD II and scribed in SAE J1979 was short and de-signed to provide critical system data only. The useful types of data we can re-trieve from OBD II generic include
short-term and long-term fuel trim val-ues, oxygen sensor voltages, engine and intake air temperatures, MAF or MAP values, rpm, calculated load, spark tim-ing and diagnostic trouble code (DTC) count. Freeze frame data and readiness status also are available in OBD II generic mode. A generic scan tool also should be able to erase trouble codes and freeze frame data when command-ed to do so.
Data coming to the scan tool through the mandated OBD II generic interface may not arrive as fast as data sent over one of the dedicated data link connec-tor (DLC) terminals. The vehicle man-ufacturer has the option of using a faster data transfer speed on other DLC pins. Data on the generic interface also may not be as complete as the informa-tion you’ll get on many
manufacturer-OBDII
GENERIC
PID
DIAGNOSIS
B
Y
K
ARL
S
EYFERT
A wealth of diagnostic information is
available on late-model OBD II-compliant
vehicles, even when ‘enhanced’ or
‘manufacturer-specific’ PIDs are not
accessible. It doesn’t take much to use
this information to its best advantage.
Phot
o: K
arl Seyf
53 September 2007
specific or enhanced interfaces. For ex-ample, you may see an engine coolant temperature (ECT) value in degrees on the OBD II generic parameter identi-fication (PID) list. A manufacturer-specific data list may display ECT status in Fahrenheit or Celsius and add a sep-arate PID for the ECT signal voltage. In spite of these and other limitations, OBD II generic mode still contains many of the trouble codes, freeze frame data and basic datastream information needed to solve many emissions-related issues.
There are nine modes of operation described in the original J1979 OBD II standard. They are:
Mode 1: Show current data Mode 2: Show freeze frame data Mode 3: Show stored trouble codes Mode 4: Clear trouble codes and stored values
Mode 5: Test results, oxygen sensors Mode 6: Test results, noncontinuously monitored
Mode 7: Show pending trouble codes Mode 8: Special control mode Mode 9: Request vehicle information
Modes 1 and 2 are basically identical. Mode 1 provides current information, Mode 2 a snapshot of the same data taken at the point when the last diag-nostic trouble code was set. The excep-tions are PID 01, which is available only
in Mode 1, and PID 02, available only in Mode 2. If Mode 2 PID 02 returns zero, then there’s no snapshot and all other Mode 2 data is meaningless. Vehi-cle manufacturers are not required to support all modes. Each manufacturer may define additional modes above Mode 9 for other information.
Most vehicles from the J1979 era sup-ported 13 to 20 parameters. The recent phase-in of new parameters will make OBD II generic data even more valu-able. The California Air Resources Board (CARB) revisions to OBD II CAN-equipped vehicles have increased the number of potential generic param-eters to more than a hundred. Not all vehicles will support all PIDs, and there are many manufacturer-defined PIDs that are not included in the OBD II standard. Even so, the quality and quantity of data have increased signifi-cantly. For more information on the new PIDs that were added to 2004 and later CAN-equipped vehicles, refer to Bob Pattengale’s article “Interpreting Gener-ic Scan Data” in the March 2005 issue of MOTOR. A PDF copy of the article can be downloaded at www.motor.com.
Establish a Baseline
If you’re repairing a vehicle that has stored one or more DTCs, make sure you collect the freeze frame data before erasing the stored codes. This data can
OBD II GENERIC PID DIAGNOSIS
Here’s a basic scanner display showing OBD II generic PIDs. Slow-changing PIDs like IAT and ECT can be fol-lowed fairly easily in this format, but it’s difficult to spot glitches in faster moving PIDs like Spark Advance.
This scan tool also allows the user to graph some PIDs, while continuing to display the others in conventional numeric format. Due to OBD II’s refresh capabilities on some vehicles, it’s best to limit your PID choices to those directly related to your diagnostic approach.
This photo illustrates how far PID data collection and display have come. Several hundred thousand techs are still using the original Snap-on “brick” (on the left), which displays a limited amount of PID data on its screen. Scrolling up or down revealed more PIDs. The color version on the right brought graphing capability to the brick, and extended the product’s life span by several years.
Phot os: K arl Seyf ert Phot o court es y Snap-on Diagnos tics
be used for comparison after your repairs. The “before” freeze frame shot and its PID data establish the baseline.
As you begin your diagno-sis, correct basic problems first—loose belts, weak bat-teries, corroded cables, low coolant levels and the like. The battery and charging sys-tem are especially important, due to their effect on vehicle electronics. A good battery, a properly functioning alterna-tor and good connections at power and ground circuits are essential. You can’t as-sume that OBD II will detect a voltage supply problem that can affect the entire system. If you have an intermittent problem that comes and goes, or random problems
that don’t follow a logical pattern, check the grounds for the PCM and any other controller in the vehicle.
If the basics check out, focus your di-agnosis on critical engine parameters and sensors first. Write down what you find; there’s too much information to keep it all in your head. Add any infor-mation collected from the vehicle own-er regarding vehicle pown-erformance. Jot
down the battery voltage and the results of any simple tests, such as fuel pressure or engine vacuum. Look at the Readi-ness Status display to see if there are any monitors that aren’t running to completion.
Datastream Analysis
Take your time when you begin looking at the live OBD II datastream. If you se-lect too many items at one time, the scan tool update will slow. The more PIDs you select, the slower the update rate will be. Look carefully at the PIDs and their values. Is there one line of data that seems wrong? Compare data items to one another.
Do MAP and BARO agree key on, engine off (KOEO)? Are IAT and ECT the same when the engine is cold KOEO? The ECT and IAT should be within 5°F of each other. ECT should reach operating temperature, preferably 190°F or higher. If the ECT is too low, the PCM may richen the fuel mixture to compensate for a (perceived) cold-engine condition. IAT should read ambi-ent temperature or close to underhood temperature, depending on the location of the sensor.
Is the battery voltage good KOEO? Is the charging voltage adequate when the engine starts? Do the MAP and BARO readings seem logical? Do the
IAC counts look too high or too low? Compare data items to known-good values you’d expect to see for similar op-erating conditions on similar vehicles.
Check short-term fuel trim (STFT) and long-term fuel trim (LTFT). Fuel trim is a key diagnostic parameter and tells you what the com-puter is doing to control fuel delivery and how the adap-tive strategy is operating. STFT and LTFT are ex-pressed as a percentage, with the ideal range being within ±5%. Positive fuel trim per-centages indicate that the powertrain control module (PCM) is attempting to en-richen the fuel mixture to compensate for a perceived lean condition. Negative fuel trim per-centages indicate that the PCM is at-tempting to enlean the fuel mixture to compensate for a perceived rich condi-tion. STFT will normally sweep rapidly between enrichment and enleanment, while LTFT will remain more stable. If either STFT or LTFT exceeds ±10%, this should alert you to a potential problem.
56 September 2007
OBD II GENERIC PID DIAGNOSIS
The Snap-on MODIS is a combination scanner, lab/igni-tion scope, DVOM and Troubleshooter. In scanner mode, MODIS can graph several parameters simultaneously, as seen in this screen capture. Remember, although these may look like scope patterns, the reporting rate for PID data on a scanner isn’t nearly as fast.
When scan tool screen real estate is limited, porting the scan tool into a laptop or desktop PC allows you to graph more PIDs simultaneously. The PC’s much larger memory ca-pacity also makes it possible to col-lect PID data in movie format for later playback and analysis.
An on-screen description of the PID displayed below the graphing data may help you to understand what you’re looking at, and avoid misunder-standings with measurement units.
Scr een captur e: Jor g e Menchu Phot o court es y SP X/O T C Phot o court es y Inject oclean
Determine if the condition exists in more than one operating range. Check fuel trim at idle, at 1500 rpm and at 2500 rpm. If LTFT B1 is 20% at idle but corrects to 5% at both 1500 and 2500 rpm, focus your diagnosis on factors that can cause a lean condition at idle, such as a vacuum leak. If the condition exists in all rpm ranges, the cause is more like-ly to be fuel-related, such as a bad fuel pump, restricted injectors, etc.
Fuel trim can also be used to identify which bank of cylinders is causing a problem on bank-to-bank fuel control engines. For example, if LTFT B1 is 25% and LTFT B2 is 5%, the source of the problem is associated with B1 cylinders only, and your diagnosis should focus on factors related to B1 cylinders only.
The following parameters could af-fect fuel trim or provide additional diag-nostic information. Also, even if fuel trim is not a concern, you might find an indication of another problem when re-viewing these parameters:
Fuel System 1 Status and Fuel Sys-tem 2 Status should be in closed-loop (CL). If the PCM is not able to achieve CL, the fuel trim data may not be accurate.
If the system includes one, the mass airflow (MAF) sensor measures the amount of air flowing into the engine.
The PCM uses this information to cal-culate the amount of fuel that should be delivered to achieve the desired air/fuel mixture. Check the MAF sensor for ac-curacy in various rpm ranges, including wide-open throttle (WOT), and com-pare it with the manufacturer’s recom-mendations.
When checking MAF sensor
read-ings, be sure to identify the unit of mea-surement. The scan tool may report the information in grams per second (gm/S) or pounds per minute (lb/min). Some technicians replace the sensor, only to realize later that the scan tool was not set correctly. Some scan tools let you change the units of measurement for different PIDs so the scan tool matches the specification in your reference man-ual. Most scan tools let you switch easily between Fahrenheit and Celsius tem-perature scales, for example. But MAF specs can be confusing when the scan tool shows lb/min and we have a spec for gm/S. Here are a few common con-version formulas, in case your scan tool doesn’t support all of these units of measurement:
Degrees Fahrenheit 32 5/9 Degrees Celsius Degrees Celsius 9/5 + 32 Degrees Fahrenheit lb/min 7.5 gm/S
gm/S 1.32 lb/min
The Manifold Absolute Pressure (MAP) Sensor PID, if available, indi-cates manifold pressure, which is used by the PCM to calculate engine load. The reading is normally displayed in inches of mercury (in./Hg). Don’t con-fuse the MAP sensor parameter with in-take manifold vacuum; they’re not the same. Use this formula: barometric
OBD II GENERIC PID DIAGNOSIS
Graphs aren’t the only way to display PID data. Once transferred to the PC with its greater screen real es-tate, PID data can be converted to formats that relate to the data. A red thermometer scale is much easier to follow than changing numbers on a scan tool.
PC-based scan tools excel at capturing and displaying large amounts of PID data for later analysis. Graphing the data, then analyzing it on-screen, may allow you to spot inconsistencies and provides an easy method for overlaying similar or related PID data.
Here’s a peek at some of the addition-al PID data that’s available on late-model vehicles. This screen capture was taken from a CAN-enabled 2005 vehicle, and includes PIDs for EVAP PURGE, FUEL LEVEL and WARM-UPS, as well as familiar PIDs like BARO. This much PID data in generic mode should aid in diagnosis when manufacturer-specific PID data is not available.
Scr een captur es: Jor g e Menchu Scr een captur e court es y Bosch Diagnos tics
pressure (BARO) MAP intake manifold vacuum. For example, BARO (27.5 in./Hg) MAP (10.5) intake manifold vacuum (17.0 in./Hg). Some vehicles are equipped with only a MAF sensor, some have only a MAP sensor and some are equipped with both.
The PIDs for Oxygen Sensor Output Voltage B1S1, B2S1, B1S2, etc., are used by the PCM to control fuel mix-ture and to detect catalytic converter degradation. The scan tool can be used to check basic sensor operation. The sensor must exceed .8 volt and drop be-low .2 volt, and the transition from be-low to high and high to low should be quick. A good snap throttle test will verify the sensor’s ability to achieve the .8 and .2 voltage limits. If this method doesn’t work, use a bottle of propane to manu-ally richen the fuel mixture to check the oxygen sensor’s maximum voltage out-put. To check the sensor’s low voltage range, simply create a lean condition and check the voltage.
Remember, your scan tool is not a lab scope. You’re not measuring the sensor in real time. The PCM receives the data from the oxygen sensor, processes it, then reports it to the scan tool. Also, a fundamental OBD II generic limitation is the speed at which that data is deliv-ered to the scan tool. In most cases, the fastest possible data rate is approximate-ly 10 times a second, with onapproximate-ly one pa-rameter selected. If you’re requesting and/or displaying 10 parameters, this slows the data sample rate, and each pa-rameter is reported to the scan tool just once per second. You can achieve the best results by graphing or displaying data from each oxygen sensor separately. If the transition seems slow, the sensor should be tested with a lab scope to veri-fy the diagnosis before you replace it.
The Engine Speed (RPM) and Igni-tion Timing Advance PIDs can be used to verify good idle control strategy. Again, these are best checked using a graphing scan tool. Check the RPM,
Vehicle Speed Sensor (VSS) and Throt-tle Position Sensor (TPS) PIDs for ac-curacy. These parameters can also be used as reference points to duplicate symptoms and locate problems in recordings.
Most PID values can be verified by a voltage, frequency, temperature, vacuum or pressure test. Engine coolant temperature, for example, can be verified with a noncontact temper-ature tester, while intake manifold vac-uum can be verified with an accurate vacuum gauge. Electrical values also should be tested with a DVOM. If the electrical value exists at the sensor but not at the appropriate PCM terminal, then the component might be experi-encing a circuit fault.
Calculated Values
Calculated scan tool values can cause a lot of confusion. The PCM may detect a failed ECT sensor or circuit and store a DTC. Without the ECT sensor input,
59 September 2007
the PCM has no idea what the coolant temperature really is, so it may “plug in” a temperature it thinks will work to keep the engine running long enough to get it to a repair shop. When it does this, your scanner will display the fail-safe value. You might think it’s a live val-ue from a working sensor, when it isn’t.
Also be aware that when a compo-nent such as an oxygen sensor is discon-nected, the PCM may substitute a de-fault value into the datastream displayed on the scan tool. If a PID is static and doesn’t track with engine operating con-ditions, it may be a default value that merits further investigation.
Graphing Data
If you’ve ever found it difficult to com-pare several parameters at once on a small scan tool screen, graphing PIDs is an appealing proposition. Graphing multiple parameters at the same time can help you compare data and look for individual signals that don’t match up to actual operating conditions.
Although scan tool graphing isn’t equivalent in quality and accuracy to a lab scope reading, it can provide a com-parative analysis of the activity in the two, three, four or six oxygen sensors found in most OBD II systems.
Many scan tools are capable of stor-ing a multiple-frame movie of selected PIDs. The scan tool can be programmed to record a movie after a specific DTC is stored in the PCM. Alternatively, the scan tool movie might be triggered manually when a driveability symptom occurs. In either case, you can observe the data or download it and print it lat-er. Several software programs let you download a movie, then plot the values in a graphical display on your computer monitor.
Make the Most of
What You’ve Got
Take the time to learn what your scan tool will do when connected to a spe-cific make or model. Do your best to gather all relevant information about the vehicle system being tested. That way you can get the most out of what the scan tool and PCM have to offer. The OBD II system won’t store a DTC unless it sees (or thinks it sees) a prob-lem that can result in increased emis-sions. The only way to know what the PCM sees (or thinks it sees) is to look through the window provided by the scan tool interface.
You have a DTC and its definition. You have freeze frame data that may help you zero in on the affected compo-nent or subsystem. PIDs have already provided you with additional clues about the operation of critical sensors. Keep your diagnosis simple as long as you can. Now fix the car.
OBD II GENERIC PID DIAGNOSIS
Visit www.motor.comto download a free copy of this article.
Circle #32
Circle #33
Circle #34 Circle #35
Phot o & scr een captur es: Bob P a tt engale 52 March 2005
I
f you don’t have a good starting point, driveability diagnostics
can be a frustrating experience. One of the best places to start
is with a scan tool. The question asked by many is, “Which
scan tool should I use?” In a perfect world with unlimited
re-sources, the first choice would probably be the factory scan tool.
Unfortunately, most technicians don’t have extra-deep pockets. That’s why my first choice is an OBD II generic scan tool. I’ve found that ap-proximately 80% of the driveability problems I diagnose can be narrowed down or solved using nothing more than OBD II generic parameters. And all of that information is available on an OBD II generic scan tool that can be purchased for under $300.
The good news is the recent phase-in
of new parameters will make OBD II generic data even more valuable. Fig. 1 on page 54 was taken from a 2002 Nis-san Maxima and shows the typical para-meters available on most OBD II-equipped vehicles. As many as 36 para-meters were available under the original OBD II specification. Most vehicles from that era will support 13 to 20 para-meters. The California Air Resources Board (CARB) revisions to OBD II CAN-equipped vehicles will increase
the number of potential generic para-meters to more than 100. Fig. 2 on page 56 shows data from a CAN-equipped 2005 Dodge Durango. As you can see, the quality and quantity of data has in-creased significantly. This article will identify the parameters that provide the greatest amount of useful information and take a look at the new parameters that are being phased in.
No matter what the driveability is-sue happens to be, the first
parame-INTERPRETING
G E N E R I C
SCAN DATA
B
Y
B
OB
P
ATTENGALE
Readily available ‘generic’ scan data provides an
excellent foundation for OBD II diagnostics.
Recent enhancements have increased the value of
ters to check are short-term fuel trim (STFT) and long-term fuel trim (LTFT). Fuel trim is a key diagnostic parameter and your window into what the computer is doing to control fuel delivery and how the adaptive strategy is operating. STFT and LTFT are ex-pressed as a percentage, with the ideal range being within 5%. Positive fuel trim percentages indicate that the powertrain control module (PCM) is attempting to enrichen the fuel mix-ture to compensate for a perceived lean condition. Negative fuel trim percentages indicate that the PCM is attempting to enlean the fuel mixture to compensate for a perceived rich condition. STFT will normally sweep rapidly between enrichment and en-leanment, while LTFT will remain more stable. If STFT or LTFT ex-ceeds 10%, this should alert you to a potential problem.
The next step is to determine if the condition exists in more than one
op-erating range. Fuel trim should be checked at idle, at 1500 rpm and at 2500 rpm. For example, if LTFT B1 is 25% at idle but corrects to 4% at both 1500 and 2500 rpm, your diagnosis should focus on factors that can cause a lean condition at idle, such as a vacu-um leak. If the condition exists in all rpm ranges, the cause is more likely to be fuel supply-related, such as a bad fuel pump, restricted injectors, etc.
Fuel trim can also be used to identi-fy which bank of cylinders is causing a problem. This will work only on bank-to-bank fuel control engines. For ex-ample, if LTFT B1 is 20% and LTFT B2 is 3%, the source of the problem is associated with B1 cylinders only, and your diagnosis should focus on factors related to B1 cylinders only.
The following parameters could af-fect fuel trim or provide additional diagnostic information. Also, even if fuel trim is not a concern, you might find an indication of another problem
when reviewing these parameters: Fuel System 1 Status and Fuel System 2 Status should be in closed-loop (CL). If the PCM is not able to achieve CL, the fuel trim data may not be accurate.
Engine Coolant Te m p e r a t u re (ECT) should reach operating temper-ature, preferably 190°F or higher. If the ECT is too low, the PCM may richen the fuel mixture to compensate for a (perceived) cold engine condition. Intake Air Temperature (IAT) should read ambient temperature or close to underhood temperature, de-pending on the location of the sensor. In the case of a cold engine check— Key On Engine Off (KOEO)—the ECT and IAT should be within 5°F of each other.
The Mass Airflow (MAF) Sensor, if the system includes one, measures the amount of air flowing into the en-gine. The PCM uses this information to calculate the amount of fuel that
54 March 2005
INTERPRETING GENERIC SCAN DATA
should be delivered, to achieve the desired air/fuel mixture. The MAF sensor should be checked for accura-cy in various rpm ranges, including wide-open throttle (WOT), and com-pared with the manufacturer’s recom-mendations. Mark Warren’s Dec. 2003 Driveability Corner column cov-ered volumetric efficiency, which should help you with MAF diagnos-tics. A copy of that article is available at www.motor.com, and an updated volumetric efficiency chart is available at www.pwrtraining.com.
When checking MAF sensor read-ings, be sure to identify the unit of measurement. The scan tool may re-port the information in grams per sec-ond (gm/S) or pounds per minute (lb/min). For example, if the MAF sensor specification is 4 to 6 gm/S and your scan tool is reporting .6 lb/min, change from English units to metric units to obtain accurate readings. Some technicians replace the sensor, only to realize later that the scan tool was not set correctly. The scan tool manufacturer might display the
para-meter in both gm/S and lb/min to help avoid this confusion.
The Manifold Absolute Pressure (MAP) Sensor, if available, measures manifold pressure, which is used by the PCM to calculate engine load. The reading in English units is normally displayed in inches of mercury (in./Hg). Don’t confuse the MAP sen-sor parameter with intake manifold vacuum; they’re not the same. A sim-ple formula to use is: barometric pres-sure (BARO) MAP intake mani-fold vacuum. For example, BARO 27.5 in./Hg MAP 10.5 intake manifold vacuum of 17.0 in./Hg. Some vehicles are equipped with only a MAF sensor, some have only a MAP sensor and some are equipped with both sensors.
Oxygen Sensor Output Voltage B1S1, B2S1, B1S2, etc., are used by the PCM to control fuel mixture. An-other use for the oxygen sensors is to detect catalytic converter degradation. The scan tool can be used to check ba-sic sensor operation. Another way to test oxygen sensors is with a graphing
scan tool, but you can still use the data grid if graphing is not available on your scanner. Most scan tools on the market now have some form of graph-ing capability.
The process for testing the sensors is simple: The sensor needs to exceed .8 volt and drop below .2 volt, and the transition from low to high and high to low should be quick. In most cases, a good snap throttle test will verify the sensor’s ability to achieve the .8 and .2 voltage limits. If this method does not work, use a bottle of propane to manually richen the fuel mixture to check the oxygen sensor’s maximum output. To check the low oxygen sensor range, simply create a lean condition and check the voltage. Checking oxygen sensor speed is where a graphing scan tool helps. Fig. 3 on page 57 and Fig. 4 on page 58 show examples of oxygen sensor data graphed, along with STFT, LTFT and rpm, taken from two different graph-ing scan tools.
Remember, your scan tool is not a lab scope. You’re not measuring the
INTERPRETING GENERIC SCAN DATA
sensor in real time. The PCM re-ceives the data from the oxygen sen-sor, processes it, then reports it to the scan tool. Also, a fundamental OBD II generic limitation is the speed at which that data is delivered to the scan tool. In most cases, the fastest possible data rate is approximately 10 times a second with only one parame-ter selected. If you’re requesting and/or displaying 10 parameters, this slows the data sample rate, and each parameter is reported to the scan tool just once per second. You can achieve the best results by graphing or dis-playing data from each oxygen sensor separately. If the transition seems slow, the sensor should be tested with a lab scope to verify the diagnosis be-fore you replace it.
Engine Speed (RPM) and Igni-tion Timing Advance can be used to verify good idle control strategy. Again, these are best checked using a graphing scan tool.
The RPM, Vehicle Speed Sensor (VSS) and Throttle Position Sensor (TPS) should be checked for accuracy.
These parameters can also be used as reference points to duplicate symptoms and locate problems in recordings.
Calculated Load, MIL Status, Fuel Pressure and Auxiliary Input Status (PTO) should also be consid-ered, if they are reported.
Additional OBD II
Parameters
Now, let’s take a look at the more re-cently introduced OBD II parameters. These parameters were added on 2004 CAN-equipped vehicles, but may also be found on earlier models or non-CAN-equipped vehicles. For example, the air/fuel sensor parameters were available on earlier Toyota OBD II ve-hicles. Fig. 2 was taken from a 2005 Dodge Durango and shows many of the new parameters. Parameter de-scriptions from Fig. 2 are followed by the general OBD II description: FUEL STAT 1 Fuel System 1 Status: Fuel system status will display more than just Closed Loop (CL) or Open Loop (OL). You might find one
of the following messages: OL-Drive, indicating an open-loop condition during power enrichment or decelera-tion enleanment; OL-Fault, indicating the PCM is commanding open-loop due to a system fault; CL-Fault, indi-cating the PCM may be using a differ-ent fuel control strategy due to an oxygen sensor fault.
ENG RUN TIME Time Since En-gine Start: This parameter may be useful in determining when a particu-lar problem occurs during an engine run cycle.
DIST MIL ON Distance Traveled While MIL Is Activated: This para-meter can be very useful in determin-ing how long the customer has al-lowed a problem to exist.
COMMAND EGR EGR_PCT: Commanded EGR is displayed as a percentage and is normalized for all EGR systems. EGR commanded OFF or Closed will display 0%, and EGR commanded to the fully open
57 March 2005
position will display 100%. Keep in mind this parameter does not reflect the quantity of EGR flow—only what the PCM is commanding.
EGR ERROR EGR_ERR: This parameter is displayed in percentage and represents EGR position errors. The EGR Error is also normalized for all types of EGR systems. The reading is based on a simple formula: (Actual EGR Position Commanded EGR) Commanded EGR EGR Error. For example, if the EGR valve is command-ed open 10% and the EGR valve moves only 5% (5% 10%) 10% 50% error. If the scan tool displays EGR Er-ror at 99.2% and the EGR is command-ed OFF, this indicates that the PCM is receiving information that the EGR valve position is greater than 0%. This may be due to an EGR valve that is stuck partially open or a malfunctioning EGR position sensor.
EVAP PURGE EVAP_PCT: This parameter is displayed as a percent-age and is normalized for all types of purge systems. EVAP Purge Control
commanded OFF will display 0% and EVAP Purge Control commanded fully open will display 100%. This is an important parameter to check if the vehicle is having fuel trim prob-lems. Fuel trim readings may be ab-normal, due to normal purge opera-tion. To eliminate EVAP Purge as a potential contributor to a fuel trim problem, block the purge valve inlet to the intake manifold, then recheck fuel trim.
FUEL LEVEL FUEL_PCT: Fuel level input is a very useful parameter when you’re attempting to complete system monitors and diagnose specif-ic problems. For example, the misfire monitor on a 1999 Ford F-150 re-quires the fuel tank level to be greater than 15%. If you’re attempt-ing to duplicate a misfire condition by monitoring misfire counts and the fuel level is under 15%, the misfire moni-tor may not run. This is also impor-tant for the evaporative emissions monitor, where many manufacturers require the fuel level to be above 15% and below 85%.
WARM-UPS WARM_UPS: This parameter will count the number of warm-ups since the DTCs were cleared. A warm-up is defined as the ECT rising at least 40°F from engine starting tem-perature, then reaching a minimum temperature of 160°F. This parameter will be useful in verifying warm-up cy-cles, if you’re attempting to duplicate a specific code that requires at least two warm-up cycles for completion. BARO BARO: This parameter is useful for diagnosing issues with MAP and MAF sensors. Check this parameter KOEO for accuracy relat-ed to your elevation.
C AT T M P B 1 S 1 / B 2 S 1 CATEMP11, 21, etc.: Catalyst tem-perature displays the substrate temper-ature for a specific catalyst. The tem-perature value may be obtained directly from a sensor or inferred using other sensor inputs. This parameter should have significant value when checking catalyst operation or looking at reasons for premature catalyst failure, say, due to overheating.
INTERPRETING GENERIC SCAN DATA
CTRL MOD (V) VPWR: I was surprised this parameter was not in-cluded in the original OBD II specifi-cation. Voltage supply to the PCM is critical and is overlooked by many technicians. The voltage displayed should be close to the voltage present at the battery. This parameter can be used to look for low voltage supply is-sues. Keep in mind there are other voltage supplies to the PCM. The igni-tion voltage supply is a common source of driveability issues, but can still be checked only with an enhanced scan tool or by direct measurement. ABSOLUT LOAD LOAD_ABS: This parameter is the normalized value of air mass per intake stroke displayed as a percentage. Absolute load value ranges from 0% to approximately 95% for normally aspirated engines and 0% to 400% for boosted engines. The infor-mation is used to schedule spark and EGR rates, and to determine the pumping efficiency of the engine for di-agnostic purposes.
OL EQ RATIO EQ_RAT: Com-manded equivalence ratio is used to de-termine the commanded air/fuel ratio of the engine. For conventional oxygen sensor vehicles, the scan tool should dis-play 1.0 in closed-loop and the
PCM-commanded EQ ratio during open-loop. Wide-range and linear oxygen sensors will display the PCM-com-manded EQ ratio in both open-loop and closed-loop. To calculate the actual A/F ratio being commanded, multiply the stoichiometric A/F ratio by the EQ ratio. For example, stoichiometric is a 14.64:1 ratio for gasoline. If the com-manded EQ ratio is .95, the command-ed A/F is 14.64 0.95 13.9 A/F. TP-B ABS, APP-D, APP-E, COM-MAND TAC: These parameters relate to the throttle-by-wire system on the 2005 Dodge Durango of Fig. 2 and will be useful for diagnosing issues with this system. There are other throttle-by-wire generic parameters available for differ-ent types of systems on other vehicles.
There are other parameters of inter-est, but they’re not displayed or avail-able on this vehicle. Misfire data will be available for individual cylinders, similar to the information displayed on a GM enhanced scan tool. Also, if available, wide-range and linear air/fuel sensors are reported per sensor in voltage or milliamp (mA) measurements.
Fig. 5 above shows a screen capture from the Vetronix MTS 3100 Mas-tertech. The red circle highlights the “greater than” symbol (>), indicating that multiple ECU responses differ in
value for this parameter. The blue cir-cle highlights the equal sign (=), indi-cating that more than one ECU sup-ports this parameter and similar values have been received for this parameter. Another possible symbol is the excla-mation point (!), indicating that no re-sponses have been received for this parameter, although it should be sup-ported. This information will be useful in diagnosing problems with data on the CAN bus.
As you can see, OBD II generic data has come a long way, and the data can be very useful in the diagnostic process. The important thing is to take time to check each parameter and determine how they relate to one another.
If you haven’t already purchased an OBD II generic scan tool, look for one that can graph and record, if pos-sible. The benefits will immediately pay off. The new parameters will take some time to sort out, but the diag-nostic value will be significant. Keep in mind that the OBD II generic specification is not always followed to the letter, so it’s important to check the vehicle service information for variations and specifications.
60 March 2005
INTERPRETING GENERIC SCAN DATA
Fig. 5
Visit www.motor.com to download a free copy of this article.
L
ast month’s installment on datastream analysis focused on the value of freeze frame data, Mode 5 and Mode 6 data and KOEO (key on, engine off) datastream. This month’s discussion picks up where we left off, with KOER (key on, engine running) analysis. So go ahead, start the engine!I recommend that KOER data col-lection always start in the generic, or global OBD II interface. Why? Because generic datastream PID values are nev-er substitutes for actual sensor readings. For example, you can disconnect the MAP sensor connector on a Chrysler
product and drive it around while moni-toring datastream in the enhanced (manufacturer-specific) interface. (Try this yourself; don’t just take my word for it.) You’ll see the MAP PID change along with the TPS sensor reading and rpm, showing a range of values that re-flect likely MAP readings for each con-dition, moment by moment. These are substituted values. If you looked at the MAP voltage PID, however, it would show an unchanging reference voltage. In the enhanced interface, substitutions can and do occur. But in the generic in-terface, substituted values are never al-lowed. You would see MAP shown at a constant pressure equal to something a
DATASTREAM
IN-DEPTH
ANALYSIS
B
Y
S
AM
B
ELL
We began this two-part article with a
discussion of preliminary OBD II datastream
analysis, conducted with the engine off.
We’re going deeper this time, to explain
the value of datastream information
collected with the engine running.
Phot
oillus
tr
ation: Harold P
erry; phot
os: Wieck Media & Jupit
er Imag
bit higher than BARO. The generic in-terface allows calculated values, but never substituted values.
So, what are we looking for, now that we’ve finally started the engine? The specific answer, of course, will depend largely on the details of the customer complaint and/or DTC(s) that are stored. We might, for example, be focus-ing on fuel trim numbers (and trends) if our code suggests an underlying air/fuel metering problem. We might be looking most closely at engine coolant tempera-ture, and time-until-warm measure-ments when that seems warranted. Per-haps our problem lies in the evap area, or involves EGR flow. But ultimately, it doesn’t matter what the specific issue is; we’ll have to focus in on the systemic in-teractions that determine the overall characteristics of a particular data set.
Here’s a concrete example to illus-trate what I mean. The vehicle in ques-tion is a 1999 Chevy Venture minivan with the 3.4L V6. There was a DTC
P0171 (Exhaust Too Lean, Bank 1) in memory with an active MIL. The sum of Short Term and Long Term Fuel Trims in freeze frame was in excess of
50%. Fuel pressure and volume had been verified as within specification.
When evaluating a fuel trim trouble code, one of the first steps must always be to verify that the oxygen sensor (on which the DTC is based) is functioning correctly. During the test drive, I ob-served the O2sensor switching rich, but
not as often as would be expected if the very large fuel trim corrections shown were actually effective. Indeed, on the face of it, datastream seemed to confirm the DTC. Longtime readers, however, can probably anticipate what my next tests were: I checked the actual lambda value of the exhaust gases. Then I looked for a dynamic response as I artifi-cially enriched the system with a blast of propane, then enleaned it by discon-necting a major vacuum hose. (See “What Goes In…Harnessing Lambda as a Diagnostic Tool” in the September 2005 issue of MOTOR. Search the index at www.motormagazine.com for all MO
-TORmagazine articles mentioned.)
Hav-38 August 2008
DATASTREAM IN-DEPTH ANALYSIS
Data collection and analysis might yield some helpful information, if you can find the wheat within the chaff. This is only a small portion of a larger data set with 100 values per PID.
When evaluating a
fuel trim trouble
code, one of the
first steps must
always be to verify
that the oxygen
sensor (on which the
DTC is based) is
functioning correctly.
Chart & scr
een captur
ing found the idle lambda at a ridicu-lously low value of .85 (indicating a mix-ture with 15% more fuel than needed), I was not surprised to see that the O2
sen-sor didn’t register a rich condition until the engine was very nearly flooded with propane. When I removed the purge hose, engine rpm climbed and the en-gine smoothed out, while lambda marched toward the stoichiometric ideal value of 1.00. Once the faulty O2sensor
was replaced, all aspects of driveability improved, and the minivan returned to its previous fuel consumption levels.
Dynamic tests verify DTC accuracy. In some instances, we may be able to utilize bidirectional controls embedded within our scan tool packages to actuate various components. In other cases, we may need to improvise, using signal simulators, power probes, jumpers, propane or just good, old-fashioned test driving as required to initiate change within the system we’re working on. (I’m not saying that it will always be as
easy as it was with the Venture. You and I know there will be problems that don’t set DTCs, problems that do set DTCs that have no apparent connection to the
actual root fault and, of course, prob-lems that set appropriate codes yet are still really hard to diagnose.)
Floodlights and Spotlights
One of the most powerful features of most scan tools is, as nearly as I can tell, one of the least used. This is the so-called flight recorder, data logger or movie mode. By whatever name it’s known, this is an analytical tool of con-siderable value.
Take a look at the portion of saved scan data portrayed in the chart on page 38. As you see, any value in that infor-mation is well hidden. This might be termed a “floodlight” view, showing too many values for too many parameters. But look at the “spotlight” view above, where I’ve selected and graphed a few of the same PID values. This was a ve-hicle where there was no DTC stored in memory. By including both upstream O2sensors, I have provided myself a
cross-check, as there is less likelihood of
Graphical representations of scan data “movies” can speed analysis. As an added bonus, using your scanner’s flight recorder mode allows you to concentrate on your driving. The data set here clearly points to a lack of adequate fuel volume. This graphical representa-tion is derived from the exact same movie capture seen in the chart on the previous page.
One of the most
powerful features
of most scan tools—
the so-called flight
recorder—seems to
be one of the least
used. But it’s an
analytical tool of
considerable value.
both being bad. Similarly, MAF and rpm track nicely with one another, again providing a good cross-check. The data values at the cursor (the vertical line at frame 2) are called out at the left side of each PID’s plot. The upstream O2
sen-sors are switching nicely at 2000 rpm (as shown at frame ⫺51), but the graphic interface reveals an obvious problem at
higher speeds as the O2sensors flat-line
lean. A new fuel pump restored the missing performance.
Slow Motion and
High Speed
Moviemakers speed up or slow down the action on the screen by shooting at different numbers of frames per
sec-ond. When film shot at 20 frames per second is played back at 60 frames per second, the action seems to be occur-ring at three times the speed. Just as a 56k dial-up modem is slower than a DSL Internet connection, scan data transfer rates also vary according to the interface used. Generic communica-tion modes often travel at a crawl,
es-40 August 2008
DATASTREAM IN-DEPTH ANALYSIS
M
ost MOTORreaders have at least a passing fa-miliarity with the concept of OBD II monitor completion status. Even so, a brief refresher may be in order. OBD II monitors are simply formalized sets of self-tests all related to a particular system or component.C
Coonnttiinnuuoouuss mmoonniittoorrss..With a few very rare ex-ceptions (mostly for 1998 and earlier models), the so-called continuous monitors always show up as “complete,” “done” or “ready.” Take this status re-port with a grain of salt. Unplug the IAT sensor, start the engine and check that the “Comprehensive Component Monitor” readiness status shows com-plete. Is the MIL on? Are there any pending codes? How long would you have to let the vehicle idle be-fore it will trip the MIL and show a P0113 (IAT Sen-sor Circuit Voltage High) DTC?
As it turns out, depending on the specific make, model and powertrain package, there are several specific criteria that must be met beforethe code will set. In one instance, the PCM must detect a VSS signal of 35 mph or more and an ECT value of 140°F or more, the calculated IAT must be less than ⫺38°F and all of these conditions must be met for at least 180 seconds of continuous duration, during which no other engine DTCs are set—all while MAF is less than 12 grams per second. (This particular example, incidentally, is a two-trip code. Some other manu-facturers may make this and other DTCs under the component monitor’s jurisdiction into one- or two-trip codes, sometimes with even more complicated entry criteria.)
Continuous monitors include the comprehensive component monitor, the fuel monitor and the mis-fire monitor. Each monitor runs continuously when conditions are appropriate, but not during all actu-al driving. For example, the misfire monitor is often suspended during 4WD operation, since feedback through the axles over rough roads might cause uneven disruption of the CKP signals, which could
otherwise be misidentified as misfires. Similarly, ex-tremely low fuel tank levels may suspend both mis-fire and fuel system monitors to avoid setting a DTC for running out of gas.
N
Noonnccoonnttiinnuuoouuss mmoonniittoorrss..As I pointed out last month, it’s important to note the readiness status of the other, noncontinuous monitors as well. These are the monitors whose status will change to “incom-plete,” “not ready” or “not done” when the codes are cleared. If a vehicle arrives at your shop showing one or more incomplete monitors, it’s likely that someone has already cleared the codes beforeit got to you. (There are a few vehicles—for example, some 1996 Subarus—which may reset monitor status to in-complete at every key-off, or other vehicles which may have certain monitors which cannot be made to run to completion in normal driving, such as the evap monitor on some Toyota Paseos.) If a vehicle shows up with incomplete monitors, however, you should certainly document that fact on your work order and be sure to advise the customer that there’s a very real possibility that one or more other codes may recur af-ter the current repair has been completed. For more on this subject, see my article “How Not to Get MIL-Stoned” in the April 2004 issue of MOTOR.
More importantly, for our present purposes, the existence of incomplete monitors means that you may not be getting the whole picture as to what ails the vehicle you’re looking at. Keep an open mind, remembering that there may be other, as yet unknown issues hidden behind that incomplete monitor, and try not to rush your diagnosis. As mentioned in last month’s installment, there may
be some valuable data accessible via Mode 6 even if the monitor is not complete, but there is a very real possibility that Mode 6 data for any incom-plete monitor may turn out to be unreliable. And, of course, don’t overlook any pending DTCs. Re-member, these do not illuminate the MIL, so you must seek them out on your own.
DATASTREAM IN-DEPTH ANALYSIS
pecially in comparison to CAN speeds.If you’re stuck with a generic interface, you can often accomplish more by looking at less.
The key here is PID selection. Choose the smallest number of PIDs that will give you the information you
actually need. Three or four are usu-ally sufficient. This is your version of the filmmaker’s high-speed action trick, as you get more updates per unit time the fewer PIDs you select. With several hundred possible PIDs from which to choose, it’s just too
easy to miss an intermittent data glitch, or to drown in a sea of too much information (see “Live Data vs. ‘Live Data’” on page 44).
Most MOTORreaders are familiar
with the ways in which some of the ma-jor OEMs have organized data PIDs for display in their enhanced scan tool interfaces. Groupings such as Misfire, Driveability, Emissions, Accessories and the like are good examples of the types of data sets you may want to con-struct while analyzing different sorts of problems. Tracking down a nasty inter-mittent problem? Don’t hesitate to pare down the OEM groupings even further to speed data updates.
Code-Setting Criteria and
Operating Conditions
If we’re trying to resolve a MIL-on complaint, it’s critical that we first re-view both the exact code-setting crite-ria and the operating conditions as re-vealed in our previously recorded freeze frame data. We’ll need to drive in such a way as to complete a good “trip” so the affected monitors can run to completion. (For a more de-tailed discussion of OBD II trips and monitors, see “Monitors 1.01” on page 40.) If we fail to meet the condi-tions under which the self-test (moni-tor) will run, we cannot hope to make progress. Using the previously record-ed freeze frame parameters gives us a good general idea of the operating conditions required. Merely duplicat-ing speed, load, temperature and oth-er basic charactoth-eristics may not be enough. This is why we need to re-view and understand the details of the code-setting criteria and the monitor’s self-test strategy. For example, some monitors cannot run until others have already reached completion. A typical example would be a catalytic convert-er monitor that is suspended until the oxygen sensor monitors have run and passed.
Some trouble codes, or even pending codes, suspend multiple monitors. Oth-er vehicle faults may then go undetect-ed until all monitors can run again. A P0500 (VSS Malfunction) in a Corolla, for example, will effectively suspend even the misfire monitor.
I
t seems like a no-brainer: When you’re done with all your diagnostic tests and you’ve made the necessary repairs, you should turn off the MIL, right? That’s what your customer probably expects, and as we all know, meeting customer expectations is an important part of running a successful business.But there are often times when you should leave the MIL on. If your area uses an OBD II “plug & play” emissions test, the regulations usually require that no more than one monitor can be incomplete as of the time of testing for model year 2001 and newer vehi-cles, with no more than two incomplete monitors for 1996 to 2000 models. In some areas, retest eligibility requires that the converter monitor must show “com-plete” before a retest is valid.
If an emissions test or retest is looming in your cus-tomer’s future, you and he must work out the pros and cons of clearing the codes and resetting the itors to “incomplete.” If you clear the codes, the mon-itors will reset as well. This will require that someone will have to drive a sufficient number of monitors to completion before a retest will be valid. If local weather conditions, for example, will prevent the monitors from running in a timely way, your customer might be better off if you leave the MIL on. Then your customer would have to drive only those portions of the drive trace needed to run the monitor under which the current DTC set.
For example, if you’re in the frigid climes of an up-per Midwestern winter and a customer’s vehicle failed an emissions test because of a faulty O2sensor heater,
you’ll both be ahead if you don’t clear the code, letting it expire naturally as the heater monitor runs success-fully to completion on the next two trips. This will avoid the necessity of rerunning all the rest of the monitors. Of course, if the vehicle failed the evap mon-itor, you’ll be better off clearing the code, because pro-longed subfreezing temperatures may make running that particular monitor successfully virtually impossible for weeks at a time.
The net result is that we may have to clear the current DTCs and extinguish the MIL before our test drive can bear fruit. (But again, please be sure to read and record all the freeze frame data, the status of all monitors, the list of both current and pending DTCs and any available Mode 6 data before clearing the MIL (see “Lights Out?” on page 42).
We’ll need to drive long enough to let the monitors in question reach completion. In some cases, this may require an extended period of time. Many Ford products, for exam-ple, normally require a minimum of a six-hour cold-soak be-fore the evap monitor can run, although there may be ways to force this issue in some instances. Many Chrysler oxygen sen-sor monitors run only after engine shut-down (with key off),
so that no amount of driving will ever bring them to comple-tion. Certain monitors, and apparently even certain scan tools, may require a key-off sequence before the monitor status will update from incomplete to complete. MOTORoffers an excel-lent resource to help you understand these details—the OBD II Drive Cycle CD Version 7.0, available from your local MOTORDistributor (1-800-4A-MOTOR).
In some cases, local weather conditions may make monitor completion seem impossible until a later date, usually be-cause of ambient temperature requirements, although some-times as a result of road conditions. In most cases, however, it will still be possible to complete the monitor by running the vehicle on a lift or dynamometer. This option may occasional-ly result in setting, say, an ABS code, but most monitors can be run to completion swiftly and successfully on a lift. This option may also offer a safer, faster alternative to actual driv-ing, as trees and telephone poles are less likely to jump in front of a vehicle on a stationary lift.
44 August 2008
DATASTREAM ANALYSIS
Circle #22
I
ntermittent interruptions of sensor data can cause tricky driveability problems. Some glitches may set a DTC while others may not. While viewing datastream mayreveal an intermittent sensor problem, it should not be relied upon to do so. The is-sue, once again, is in the data rate. Even a moderately fast interface, say the 41.6 kbps (kilobytes per second) J-1850 PWM used on many Ford products, can easily miss a several-millisecond dropout if it’s not that particular PID’s turn in the data-stream. Where symptoms or DTCs point to-ward an intermittent sensor glitch, you’re probably better off breaking out your scope or graphing multimeter.
Conclusions
Proper in-depth datastream analysis can often light the way toward correct diagnosis of driveability concerns. Recording all available DTCs, pending DTCs, freeze frame data and Mode 5 and Mode 6 results before clearing any DTCs is essential. Specific setting cri-teria for each DTC are manufacturer-determined, regardless of whether the code assigned is generic or manufac-turer-specific. Freeze frame data sets can be used to recreate the operating conditions under which a previous fail-ure occurred and can help illuminate the conditions under which certain self-tests are conducted. Mode 5 and Mode 6 test results can help in analyz-ing the type and extent of certain fail-ures. KOEO datastream analysis can sometimes reveal sensor faults or ration-ality concerns that might otherwise be overlooked.
Looking at KOEO and KOER data-stream on a regular basis makes known-good values familiar. Once you know
the correct values, the conditions ac-companying problems identified by freeze frame are easier to spot. KOER data can highlight current problems,
es-pecially when used in conjunction with graphical scanner interfaces. Generic data PIDs cannot include substituted values, and so may point up faults easily overlooked in more enhanced inter-faces. Careful selection of custom-grouped PIDs can provide faster scan-ner update rates.
Pick your tools wisely. To verify hard faults, monitor datastream as you run actuator tests. Look for any mismatch between the command sent to a com-ponent and its actual response. For in-termittent problems, record and graph data. In tough cases, test circuits with your scope or meter to verify actual voltage for comparison to specs.
Used properly, these techniques will help you arrive quickly and confi-dently at an accurate diagnosis of the root cause of most driveability com-plaints. Circle #23
When trying to
resolve a MIL-on
complaint, it’s
critical to first
review the exact
code-setting
criteria and the
operating
conditions as
revealed in the
freeze frame data.
This article can be found online atO
nce in a while we may encounter a total fail-ure of a MAF sensor, one that is, perhaps, short circuited or inter-nally open. Much more common, however, are failure modes in which the MAF sensor has become un-reliable, underreporting or overreport-ing the true airflow into the engine. In-deed, as we shall see, many MAF sen-sor failures actually result in both un-derreporting and overreporting!Before we get down to brass tacks, a brief review of the basics of MAF sys-tems is in order. Fuel control syssys-tems for most modern gasoline engines are centered either on MAF or MAP (man-ifold absolute pressure). MAF systems, which, as their name suggests, measure the weight of incoming air and then meter the appropriate amount of fuel to ensure efficient combustion, are poten-tially more precise, although MAP sys-tems, which calculate fuel requirements based on engine load, have historically demonstrated greater reliability.
As you already know, combustion is most efficient when the ratio of air to fuel is approximately 14.7:1 by weight. Mass and weight are essentially synony-mous in the presence of a sufficiently strong gravitational field such as the Earth’s. Thus, knowing the weight of the air entering the engine allows the engine controller to meter the exact amount of fuel required to achieve effi-cient combustion. The controller com-mands the fuel injectors to open for an amount of time calculated to be suffi-cient to allow the correct weight of fuel to enter the engine, providing that the fuel’s pressure is known. Fuel delivery is fine-tuned by applying fuel trim correc-tions derived from the closed-loop feed-back of the oxygen sensor(s).
If the entire system is working as de-signed, fuel trim corrections, expressed as a percentage deviation from the base fuel delivery programming, will be with-in 10% (either positive or negative) of the programmed quantity. In the ab-sence of a MAF-specific diagnostic trou-ble code (DTC), what would first lead us to even suspect that a faulty MAF sensor might underlie a particular drive-ability problem?
To function correctly, all of the air
entering an engine’s combustion cham-bers must be “seen” by the MAF sen-sor. This means that any vacuum or air leak downstream of the sensor will re-sult in insufficient fuel metering, caus-ing a lean condition in open-loop opera-tion and higher-than-normal fuel trim values in closed-loop. When we en-counter a MAF sensor-equipped vehi-cle exhibiting these symptoms, we need
to check for unmetered airflow first. Remember, too, that unmetered airflow may not require an external air leak. An incorrectly applied or faulty PCV valve can result in incorrect MAF data where the PCV intake through the breather hose is upstream of the MAF.
So, the first two rules of MAF sensor diagnosis are:
1. Find and eliminate all external air
28 July 2006
MAF
DIAGNOSIS
Phot oillus tr ation b y Harold P erry; phot os court es y Wells Manufacturing Corp.
SUCCESSFUL
MAF
SENSOR
DIAGNOSIS
B
Y
S
AM
B
ELL
A broad range of seemingly unrelated or
contradictory driveability complaints
may arise from MAF sensor
performance faults. Use this guide to
navigate out of a diagnostic thicket or,
or vacuum leaks downstream of the MAF sensor. When in doubt, use a smoke machine, or lightly pressurize the intake manifold and spray with a soap & water solution.
2. Verify that the manufacturer-speci-fied PCV valve is correctly installed and functioning as designed. (This is one in-stance where precautionary replace-ment may be cost-justified.)
Only after these two steps have been completed can you safely proceed with other diagnostics. The foremost clue that the fault lies with the MAF sensor itself will be excessive fuel trim correc-tions, usually negative at idle, more or less normal in midrange operation and positive under high load conditions (see “How Contamination Affects Hot-Wire & Hot-Film MAF Sensors” on page 32).
While there are several distinct MAF sensor technologies ranging from hot-wire or hot-film to Karman vortex and Corialis sensors, and while MAF sensor outputs may take the form of variable frequency, variable current or a simple analog voltage, the diagnostic principles remain largely the same.
Let’s start with Ford vehicles, for a couple of reasons. First, they are so widespread that most of us are familiar with them. Second, most MAF sensor-equipped Ford products make use of a PID (Parameter IDentification) called BARO (barometric pressure). Up to 2001 models, this was an inferred, or calculated, value generated by the PCM (powertrain control module) in re-sponse to the maximum MAF flow rates observed on hard wide-open throttle (WOT) acceleration. Where this calculated BARO PID is available, it is of great diagnostic value, since it can confirm MAF sensor accuracy, if only under high flow rate conditions.
To use the BARO PID, you must first know your approximate local baro-metric pressure. You might consult the BARO PID on a known-good MAP sensor-equipped vehicle. Alternatively, your local airport can provide this data. Do not rely on local weather stations, however, since these usually report a “corrected” barometric pressure. If weather information is the only avail-able source, a rule of thumb is to sub-tract about 1 in. of mercury (1 in./Hg) for every 1000 ft. of elevation above sea level. This will yield a rough estimate of your actual local barometric pressure. For greater accuracy, you can purchase a functional barometer for something less than $40. Compare this data with the BARO PID. A large discrepancy here—say, more than 2 in./Hg—should direct your suspicions toward the MAF.
Confirm your hypothesis as follows: First, make sure you have followed the steps outlined in the two rules above. Next, record all freeze frame data and all DTCs, including pending DTCs. If the OBD monitor readiness status for oxygen sensors shows READY, proceed to the next step. If it doesn’t, refer to the procedures in the following para-graph now. Next, perform a KAM (Keep Alive Memory) reset and drive the vehicle. Make sure your test drive
includes at least three sustained WOT accelerations. (It’s not necessary to speed to accomplish a sustained WOT acceleration. Rather than a WOT snap from idle, an uphill downshift at 20 to 30 mph is usually sufficient. The WOT prescription can be met at throttle openings as low as 50% to 70%.) The BARO PID should update from its de-fault reading by the end of the third WOT acceleration. If it’s now close to your local barometric pressure, the MAF sensor is not likely to be faulty. If BARO is not close, try one of the clean-ing techniques explained in the sidebar “Keeping It Clean” on page 34, then again reset KAM and take a test drive. If the BARO is still out of range, a replace-ment MAF sensor is in your customer’s future. Unfortunately, in many 2002 and later Fords, the calculated BARO PID is supplanted by a direct BARO reading
taken from a sensor incorporated into the ESM (EGR System Management) valve, greatly lessening its diagnostic val-ue for our current purposes.
If the oxygen sensor monitor status showed INCOMPLETE above, you’ll have to verify O2sensor accuracy and
performance before performing the KAM reset procedure. Use a 4- or 5-gas analyzer to determine whether the air/fuel ratio is correct in closed-loop operation. The notes about lambda () below should help.
Outside of the Ford family, MAF sensor diagnosis is more difficult. Large fuel trim corrections—either positive or negative—are often the only initial pointer to MAF sensor problems. Again, any and all air leaks downstream of the MAF sensor must be repaired first. Since accurate fuel trim correc-tions depend on correct O2sensor
out-puts, you must verify the functionality of these sensors first. The easiest and fastest way to do this is by checking lambda, a type of measure of the air/fuel ratio. (For a detailed explanation, see my article in the September 2005 issue of MOTOR.) If the O2sensors are
func-tioning correctly, lambda at idle should be very nearly equal to 1.00 in closed-loop. You may wish to check this also at 1500 to 1800 rpm to verify adequate mixture control off idle. Once lambda is found to be correct, the O2sensors are
proven good. Then any fuel trim adjust-ments must result from unmetered or incorrectly metered airflow or from in-correct fuel delivery.
Distinguishing between fuel delivery problems and MAF sensor problems can be very frustrating. Start by verify-ing fuel pressure and volume. (Those who rely on pressure alone may regret
30 July 2006
SUCCESSFUL MAF SENSOR DIAGNOSIS
Fig. 1 Fig. 2
Fig. 3 Fig. 4
Scr
een captur
it.) Use your scan tool to record critical data PIDs and graph them for analysis. Here are a couple of examples:
In Fig. 1 on page 30, taken during a period of closed-loop operation, short-term fuel trims (blue and green traces) for each bank were above 13% at 1100 rpm (red trace), yet dropped sharply negative at 3600 rpm, proving that inade-quate fuel delivery was not the problem. The values indicated in the legend box-es corrbox-espond to the readings obtained
at the indicated cursor position (vertical black line). The vertical white line indi-cates the trigger point for the recording. Subsequent diagnostics focused on the MAF sensor and the PCV system.
Take a look at the scan data graph shown in Fig. 2. It shows a car whose faulty fuel pump was unable to deliver sufficient fuel under high load condi-tions. Notice the very low O2sensor
readings (displayed in blue) corre-sponding to the cursor (black vertical
line just to the right of the zero time stamp). Fuel pressure was within spec at idle and at about 2000 rpm, but vol-ume was very low. The sudden drop-off in O2activity in response to hard
acceleration is a characteristic ob-served in many instances of MAF sen-sor faults as well.
Ultimately, known-good snapshots, waveforms and other data sets are in-valuable. Take a look at the scan snap-shot in Fig 3. Does it show good fuel trim and appropriate MAF sensor readings?
Since total fuel trim stays well within the 0 ±10% range throughout the trace, it’s a good bet that the MAF sen-sor is working well, at least under the sampled conditions.
How about the data set shown in Fig. 4? In fact, the snapshot was taken during open-loop, closed-throttle de-celeration when fuel was not being in-jected, so the O2sensor PID makes
sense. It’s actually a substituted default value inserted whenever the vehicle is in closed-throttle decel mode. What about the reported MAP value? A reading of 4.00 in./Hg shows very high engine vacuum, which jibes with the reported TPS PID. The fuel trim data is within the usually accepted range of 0 ±10%. Good data can come in a vari-ety of formats.
Of course, waveform captures from your scope are often all that are needed to confirm a faulty MAF sensor. In our shop, we’ve found that a snap-throttle MAF test for Ford products should al-ways produce a peak voltage of at least 3.8 volts DC. The snap-throttle test is
SUCCESSFUL MAF SENSOR DIAGNOSIS
Fig. 5 Fig. 6
H
ot-wire and hot-film MAF sen-sors calculate airflow based on monitoring the current re-quired to maintain a constant tem-perature in the sensing element. When dirt accumulates, the addi-tional surface area allows greater heat dissipation at low airflow rates. The dirt, however, also functions as an insulator, with an overall net re-sistance to heat transfer at very high airflow rates.At idle and under relatively low flow/load rate conditions where the majority of operation may take place, the surface area effect usual-ly predominates, causing a rich con-dition with fuel trim corrections usually in the range of ⫺10% to ⫺5%. At sustained high flow/load rates, the insulative effect usually takes over, causing a lean mixture needing fuel trim corrections as high as +30%.
Worse still is a complex case of
“mass confusion” that may arise un-der hard acceleration when long-term negative fuel trim corrections, learned in closed-loop under low-flow-rate conditions, are applied precisely when positive fuel trim cor-rections would be more appropriate. So, for example, when the system goes to open-loop during hard accel-eration where the MAF is already
underreporting airflow by up to
30%, the PCM may subtract an
addi-tional 10% to 15% (LTFT) from the
normal fuel delivery calculation, leaving the system as much as 45% leaner than desired!
In midrange operation, the two effects (surface area and insulative properties) may roughly cancel each out, with fuel trims being more or less normal. Additionally, the exact chemistry and configuration of dirt buildups can vary, changing the bal-ance of power between the surface area and insulative effects.
