ENGINE TROUBLESHOOTING

The need for troubleshooting is dictated by unsatis-factory powerplant performance. Efficient trou-bleshooting is based on a systematic analysis of

Reciprocating Engine Operation, Maintenance, Inspection, and Overhaul 2-19

what is happening so you will be able to determine the cause of a malfunction. There is no magic in suc-cessful troubleshooting, but rather an application of logic and a thorough knowledge of the basics of engine operation. For example, if you are faced with a problem of deteriorating engine performance, the first thing you should do is get all of the facts. Take nothing for granted, and ask the pilot questions. For example, find out if the trouble comes about sud-denly or was it a gradual decrease in performance?

Under what conditions of altitude, humidity, tem-perature, or power setting does this performance loss show up? Does temporarily switching to one magneto cause any change in performance? What effect did leaning the mixture or applying carbure-tor heat have on the problem? Did switching from one fuel tank to another, or turning on the fuel boost pump have any effect on the problem?

After getting all of the facts, perform a ground check to see if the problem can be duplicated. The next step is to eliminate all of the areas that are not likely to cause the trouble. For example, if the magneto drop is normal, but there is a loss of power, the igni-tion system more than likely is not the problem. To assist in the troubleshooting process, some manu-facturers provide troubleshooting flow charts or trouble-cause-remedy charts. [Figure 2-24]

BACKFIRING

When an excessively lean fuel/air mixture passes into a cylinder, the mixture may not burn at all or will burn so slowly that combustion continues through the power and exhaust strokes. If this occurs, the flame can linger in the cylinder and ignite the contents of the intake manifold and the induction system when the intake valve opens. This causes an explosion known as backfiring, which can damage the carburetor and other parts of the induc-tion system.

Backfiring is seldom the fault of the carburetor and, in most cases, is limited to one or two cylinders.

Usually, backfiring is the result of incorrect valve clearance, defective fuel injector nozzles, or other conditions which result in a leaner mixture entering the cylinder. In some instances, an engine backfires in the idle range, but operates satisfactorily at medium and high power settings. The most likely cause, in this case, is an extremely lean idle fuel/air mixture. Enriching the mixture usually corrects this difficulty. Because backfiring cylinders fire inter-mittently, they typically run cooler than cylinders that are operating normally. Therefore, a backfiring cylinder can sometimes be detected by a cold cylin-der check.

AFTERFIRING

Afterfiring, sometimes called afterburning, often results when the fuel/air mixture is too rich. Overly rich mixtures, like excessively lean mixtures, also burn slowly. However, the slow burn rate of a rich mixture is due to the lack of sufficient oxygen. If an overly rich mixture burns past the power stroke and into the exhaust stroke, unburned fuel can be forced out of a cylinder into the exhausted gases. If this occurs, air from outside the exhaust stacks will mix with the unburned fuel, causing it to ignite and explode in the exhaust system. Afterfiring is per-haps more common with engines that have long exhaust ducting that can retain greater amounts of unburned fuel. Typical causes of afterfiring include an improperly adjusted carburetor or an unseated exhaust valve.

Afterfiring can also be caused by cylinders which are not firing because of faulty spark plugs, defec-tive fuel injection nozzles, or incorrect valve clear-ances. The unburned mixture from these dead cylinders passes into the exhaust system, where it ignites and burns. Unfortunately, the resulting after-burn can easily be mistaken for evidence of a rich carburetor. Cylinders which are afterfiring intermittently can cause a similar effect. Again, the malfunction can be remedied by finding the cause and correcting the defect.

COLD CYLINDER CHECK

A cold cylinder check can help determine the oper-ating characteristics of each cylinder on an engine.

The tendency of any cylinder or cylinders to be cold or only slightly warm indicates lack of combustion within the cylinder. A cold cylinder check is made with a cold cylinder indicator which is simply an accurate pyrometer with a probe that is touched to a cylinder. Engine difficulties which can be analyzed by use of the cold cylinder indicator are:

1. Rough engine operation.

2. Excessive rpm drop or intermittent misfiring dur ing the ignition system check.

3. High manifold pressure for a given engine rpm during the ground check when the propeller is in the full low pitch position.

4. Improper valve clearances.

To conduct a cold cylinder check, you must run the engine until the cylinders are warm. When doing this, it is imperative that you head the aircraft into the wind to minimize irregular cooling and to ensure even propeller loading. Once the engine is running, duplicate the conditions that produce the

2-20 Reciprocating Engine Operation, Maintenance, Inspection, and Overha ul

Figure 2-24. This trouble-cause-remedy chart lists some general conditions or troubles which may be encountered with recipro-cating engines, such as "engine fails to start." The chart then goes on to give probable causes contributing to the condition.

Corrective actions are indicated in the "remedy" column.

Reciprocating Engine Operation, Maintenance, Inspection, and Overhaul 2-21

difficulties you want to analyze. Operate the engine at its roughest speed until cylinder head tures reach approximately 300蚌, or until tempera-tures stabilize at a lower reading. Once this occurs, shut down the engine by moving the mixture to idle cut off. When the engine ceases firing, turn off the ignition and master switches.

As soon as the engine is secured, check the temper-ature of each cylinder using the cold cylinder tester.

Start with the number one cylinder and proceed in numerical order around the engine as rapidly as possible. Recheck any low readings.

In interpreting the results of a cold cylinder check, remember that the temperatures are relative and cylinder temperature taken alone means little.

However, when the temperature of one cylinder can be compared with the temperatures of other cylin-ders, cylinder problems can be identified.

COMPRESSION TESTING

A cylinder compression test determines if the valves, piston rings, and pistons are adequately seal-ing the combustion chamber. Cylinders with good compression provide the most power while cylin-ders with low compression provide minimal power.

Low compression for the most part can be traced to valves that leak because of incorrect valve clear-ances or because the valve timing is too early or too late. Several other conditions can cause leaking valves such as carbon particles between the valve face and seat or valves that have been burned or warped. In addition, low compression can result from excessive wear of piston rings and cylinder walls or pistons that have become worn, scuffed, or damaged in some way.

Before performing a compression test, you should run an engine so the piston rings, cylinder walls, and other parts are freshly lubricated. However, it is not necessary to operate an engine prior to accom-plishing a compression check during engine buildup or on individually replaced cylinders. In these cases, spray a small quantity of lubricating oil into the cylinder or cylinders before conducting the test and turn the engine over several times to seal the piston and rings in the cylinder barrel.

The two basic types of compression testers are the differential compression tester and the direct com-pression tester. A differential pressure tester checks the compression of an aircraft engine by measuring air leakage in a cylinder. Tester operation is based, in part, on Bernoulli's principle. In other words, for

a given airflow through a fixed opening, a constant pressure drop across the opening results. Any change in the speed of airflow past the opening causes a corresponding change in pressure.

Therefore, if pressurized air is supplied to a cylin-der through a pressure gauge with both intake and exhaust valves closed, the amount of air that leaks by the valves or piston rings will create a corre-sponding pressure drop at the indicator. A perfect cylinder, of course, would have no leakage and no pressure drop would occur. [Figure 2-25]

When performing a differential compression test, you must follow the aircraft manufacturer's instruc-tions. However, there are some general guidelines that apply to most tests. The following is a list of common steps taken when performing a differential compression test:

1. Remove the most accessible spark plug from the cylinder and install the compression tester

adapter in the spark plug hole.

2. By hand, rotate the engine in the direction of nor mal operation until the piston in the cylinder you are testing is at top dead center on the compres sion stroke. If you pass top center, back the pro peller up at least 180 degrees prior to turning the propeller again in the direction of rotation. This is necessary to eliminate the effect of backlash in the valve operating mechanism and to keep the piston rings seated on the lower ring lands.

3. Connect the compression tester to a 100 to 150- psi air supply. With the shutoff valve on the com pression tester closed, adjust the regulator of the compression tester to obtain 80 psi on the regu lated pressure gauge.

REGULATED CYLINDER ■>,

PRESSURE PRESSURE

Figure 2-25. A differential compression tester measures leakage in a cylinder and is a valuable diagnostic tool.

2-22 Reciprocating Engine Operation, Maintenance, Inspection, and Overhaul

4. Recheck the compression tester to verify that the shutoff valve on the tester is closed and connect the tester to the spark plug adapter. Verify that the propeller path is clear of all objects and person nel, then open the shutoff valve on the compres sion tester. If the piston is past top dead center when pressure is applied, the propeller will rotate in the direction of engine rotation.

5. With the regulated pressure at 80 psi, read the cylin der pressure gauge. If the cylinder pressure gauge reading is below the minimum specified for the engine being tested, move the propeller in the direc tion of rotation to seat the piston rings in their grooves. However, if you move the propeller while air pressure is applied to the cylinder, make sure you have a tight grip on the propeller to prevent its rota tion. Check all the cylinders and record the readings.

If low compression is indicated on any cylinder, turn the engine through with the starter or restart and run the engine to takeoff power and recheck the low cylinder. When checking the compression, lis-ten carefully to see if you can determine the source of the leakage. Air can leak from the cylinder in three places: past the intake valve, past the exhaust valve, and past the piston rings. Leakage, or blow-by past the exhaust valve is typically identified by a hissing or whistling heard at the exhaust stack. On the other hand, air leaking past the intake valve can usually be heard through the carburetor. A hissing sound in the crankcase breather indicates air leak-ing past the piston rleak-ings.

If the piston ring gaps on a piston happen to be aligned when a compression test is done, a worn or defective ring indication will result. If this happens, you should run the engine for a period of time so the ring gaps have a chance to shift.

A direct compression test indicates the actual pres-sures within the cylinder. This method is less effec-tive than the differential pressure method in discerning a particular defective component within the cylinder. However, the consistency of the read-ings provided by a direct compression test indicate an engine's condition as a whole.

In general, most engine maintenance manuals con-tain instructions on performing a direct compres-sion test. However, some general guidelines apply to most tests and are presented here:

1. Remove the most accessible spark plug from each cylinder.

2. Clear the area around the prop arc and rotate the engine with the starter to eject excess oil accu

mulations and loose carbon particles from the cylinders.

3. Install a tester in each cylinder and tighten the recommended amount. If only one tester is avail able, check each cylinder individually.

4. With the throttle open, rotate the engine at least three complete revolutions by

engaging the

starter and record the compression readings. It is advisable to use external power when cranking the engine because a weak aircraft battery may result in a slow engine-turning rate and lower than expected compression readings.

5. Re-check any cylinder that registers a compres sion value significantly lower than the other cylinders to verify accuracy. A reading approxi mately 15 psi lower than the others indicates a cylinder leak that must be repaired. To be sure the low reading is not the result of a faulty tester, repeat the compression check with a tester

known to be accurate.