Oxygen Sensor and Closed Loop Function

10.1 Overview

Your new FAST XFI comes with a wide-band oxygen sensor that will read air/fuel ratios from as rich as 9:1 to as lean as 16.0:1.

Closed loop modemeans that the ECU will compare the measured air/fuel ratio to the target air/fuel ratio and automatically increase or decrease fuel delivery accordingly. In open loop mode, the ECU will still use the oxygen sensor to read air/fuel ratios, but will not adjust fuel delivery as in closed loop mode.

10.2 Closed Loop Parameters

10.2.1 Closed Loop High RPM

This is the RPM that must be exceeded before the ECU will enter closed loop mode. This number should be set to a higher value than the value in Closed Loop Low RPM.

10.2.2 Closed Loop Low RPM

Once the ECU has entered closed loop mode, the engine RPM must drop below this number before closed loop mode is turned off.

10.2.3 Cold Closed Loop Delay

The minimum time from initial startup before entering closed loop mode when the coolant temperature is below 60 degrees F.

10.2.4 Hot Closed Loop Delay

The minimum time from initial startup before entering closed loop mode when the coolant temperature is above 60 degrees F.

10.2.5 Maximum Closed Loop Step

This parameter defines the maximum change in fuel correction (O2 Corr. (%)) allowed by the ECU from one calculation to the next. This value is typically left at 9%.

10.2.6 Closed Loop Minimum Temperature

This is the coolant temperature that must be exceeded for closed loop operation to begin.

10.2.7 Closed Loop Enable

Checking this box turns on closed loop operation if the parameters for closed loop operation are met. If this box is unchecked, the ECU will run in open loop mode.

NOTE - In addition to the parameters set here, there is one other condition which may inhibit closed loop operation. If any after-start

enrichment fuel is being applied, the ECU will not go into closed loop mode, even if all other parameters have been met. This is described in greater detail here.

10.3 Closed Loop Gain Table

In Speed/Density mode, the Closed Loop Gain Table will appear as a 3D table of intake manifold pressure in kilopascals (kPa) vs. engine RPM. The table will be labeled "Closed Loop Gain(%) vs. RPM and MAP". In Alpha-N mode, the Closed Loop Gain Table will appear as a 3D table of throttle position vs. engine RPM. The table will be labeled " Closed Loop Gain(%) vs. RPM and TPS".

This parameter affects the amount of fuel correction applied in closed loop mode, based upon the difference of the

measured air/fuel ratio from the current target air/fuel ratio. Large injectors generally require small gain numbers, and vice versa. An excellent method of determining the best number for your application is to observe the oxygen sensor

correction factor in a data log. If the amount of correction constantly swings from a positive number to a negative number, then your closed loop gain setting should be reduced until this swing is minimized. If the actual air/fuel ratio never seems to get very close to the target air/fuel ratio, you should increase your closed loop gain setting. Closed loop operation is most effective when you have the gain number set as high as possible with a minimum amount of “overshoot” in the oxygen sensor correction.

Different gains can be specified for different RPM and engine loads. This allows closed loop operation to be effective over a broad range of operating conditions. Gains are typically higher at higher RPM and higher engine load. At higher RPM, more combustion events are happening for a given amount of time. The ECU is getting feedback – from the oxygen sensor - on its fueling adjustments more quickly. This allows it to be more aggressive with its corrections with a lower risk of drastic overshoot compared to low RPM operation. Also, at high speed and load, the engine will be more likely to be in a steady state condition – which is where a closed loop system is most effective. At idle and cruise, conditions are

changing enough that you only want the ECU to gradually adjust fueling to keep it from wildly chasing a moving target.

10.4 Correction Limit Tables

These are a pair of 3D tables of closed loop correction limits in intake manifold pressure in kilopascals (kPa) vs. engine RPM. These limits determine how much oxygen sensor correction can be applied when running in closed loop mode.

Limits for positive correction (adding fuel) are entered in one table and limits for negative correction (subtracting fuel) are entered in another.

Ideally, a well-tuned engine would only ever require a small percentage of correction - somewhere around 5% in either direction. However, more correction can be allowed to compensate for varying atmospheric conditions, fuel pressure fluctuations, less than perfect base fuel table, etc.

These correction limits act as a fail safe to prevent massively rich or lean conditions in the event of an oxygen sensor failure or other anomaly. For example, imagine there is a problem with one cylinder that prevents it from firing – maybe a plug wire has come loose. The oxygen sensor reading will change but it is not a true reflection of what is happening inside the properly firing cylinders. Without correction limits in place, the ECU would apply drastic fueling changes to try to bring the oxygen sensor reading back to its target. Now instead of one problem cylinder, all of the cylinders may be in trouble.

In general, it is safer to allow more positive correction than negative correction - especially under heavy load (higher MAP or TPS values). A lean condition at high speed and load can quickly damage an engine.