February 5, 1996 How to Monitor Your Engine's Condition

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by	John Schwaner
	This article is Copyright © 1995 by Sacramento Sky Ranch Inc. All rights reserved.

About the Author ... John Schwaner is AVweb's powerplant expert. John is a world-class authority on piston aircraft engines, and a specialist in the engineering analysis of engine failures. John runs Sacramento Sky Ranch, Inc., a leading distributor of aircraft and engine parts, and probably the foremost aircraft hose shop and magneto overhaul facility in the U.S. John and his wife live in Sacramento, California. John has also written two superb technical books: Sky Ranch Engineering Manual and The Magneto Ignition System. Both can be previewed in and ordered from the AVweb Online Bookstore.

Preflight:

1. Inspect the aircraft's belly. On most aircraft, any fluid leaks from the engine compartment ends up on the belly. Fresh oil is a sign of an oil leak. Dark soot is a sign of rich engine mixture or increased combustion gas leakage past the piston rings. Fuel dye is a sign of a fuel leak. One quick look at the belly and you know whether there are any leaks in the engine compartment.

2. Take your finger tip and touch the inside edge of the exhaust pipe. If your engine's mixture and oil consumption are normal, then your finger should be clean, or possibly have a slight tan ash deposit. If your finger tip has dry black soot on it, then your engine at a rich fuel/air mixture. If your finger has oily black soot, then your engine's burning too much oil.

3. Smell inside the engine compartment for any fuel smells. Small fuel leaks evaporate fuel as they leak and may not be enough to drip. Leaks may occur at primer fittings, hose connections, or the hose itself. One sniff in the engine compartment and you've checked all of the fuel connections for leaks.

4. Check the color of the oil on the dipstick. If it looks like black lacquer then the piston rings are leaking combustion gas into the oil.

Start:

1. Listen for any out of the ordinary noises as the starter turns your engine. You should hear the starter, the clanking of the impulse couplings, and no wheezing of air out the engine breather or intake.

2. On your Continental 6-cylinder engine, does the propeller turn with the starter? If the starter turns but the propeller sometimes doesn't, then the starter adapter is slipping and needs to be repaired.

3. On Lycoming engines if the starter turns but the propeller doesn't then the starter Bendix is starting to stick. Usually cleaning and silicone spraying the starter Bendix shaft fixes the problem.

4. Does the engine kickback when starting? If it does, then you have a problem with the magneto impulse couplings, engine timing, or the starter vibrator.

5. If the engine's getting hard to start then your magnetos probably need repair.

Idle:

1. Many engine problems are first noticed during idle. Engine roughness, caused by carbon fouled spark plugs, lead fouled spark plugs, a sticky valve, or a hydraulic lifter not operating properly are more common at idle.

2. A carbon fouled spark plug clears when you increase power, a lead fouled spark plug does not clear when you increase power. A carbon fouled spark plug indicates a spark plug that is not firing constantly or that the engine is operating at a too rich fuel/air mixture. Lead fouled spark plugs indicate a rich fuel mixture or that the power is being increased too rapidly at takeoff.

3. Bad hydraulic lifters are more noticeable during idle then during flight. A worn hydraulic lifter that leaks oil causes rocker arm to valve clearance. The rocker arm strikes the valve tip instead of pushing the valve open, resulting in a tapping noise. The noise goes away as the cold engine oil flows into the hydraulic lifter. Cold oil, being more viscous, doesn't leak out the hydraulic lifter as fast as hot oil. This causes the hydraulic lifter to pump up, closing the tappet clearance and causing the tapping noise to go away. This is fine and should not be a concern if the noise goes away shortly. If tappet noise occurs regularly then replace the hydraulic lifters. Worn or defective lifters cause the valve to pound against the seat, possibly causing valve breakage.

4. Is the oil pressure at its normal position? Low oil pressure at idle and high oil pressure during flight is caused by leakage in the oil delivery system and cannot be fixed by adjusting oil pressure.

Takeoff:

1. Is takeoff rpm lower then normal? If takeoffs are getting longer and climb performance is getting worse, then suspect that a camshaft lobe is flattening out. Damaged camshaft lobes cause a gradual decrease in takeoff rpm in an otherwise smooth engine.

2. If takeoff rpm is low on a constant speed engine then the problem may be in the governor and not in the engine. Check to see if you can reach redline rpm in cruise flight. If a constant speed propeller airplane won't reach redline rpm in cruise, then the propeller governor is holding back the propeller and your

3. problem is not low engine power. In cruise flight or descent, even an engine with low power will turn a propeller past red line because of the low engine loading.

4. Monitor for engine smoothness and power.

5. Is vacuum pump pressure normal? As the vacuum pump starts to fail it often produces lower suction for a flight or two before failure.

Cruise:

1. Magneto problems often cause a slight roughness as you climb to altitude. The roughness may go away when you reduce power to cruise. High manifold pressure requires more voltage from the magneto to spark the plugs then lower manifold pressure. Therefore, if you can turn the engine roughness on and off by changing the manifold pressure, then the magneto is at fault.

2. The higher the altitude the less resistance to arching within the magneto. Therefore, a marginal magneto often causes slight engine roughness during the climb, only to clear up when you reduce power or descend to a lower altitude.

3. To some degree oil pressure follows oil temperature and oil temperature follows cylinder head temperature. As oil temperature goes up, oil pressure goes down. As cylinder head temperature goes up, oil pressure goes up. This can be used as a crosscheck of proper gauge operation. The relationship is not linear and sometimes may not exist. For example, increased heat transfer from the cylinders to the oil occurs when the piston rings start leaking hot combustion gas into the oil. This causes oil temperatures to rise without a corresponding increase in CHT temperature.

Shutdown:

1. If the propeller has more than 100 hours on it and is starting to sling oil onto the windshield then its time to send it off to a propeller shop.

2. Does the engine cutoff evenly? If not, the idle cutoff circuit is leaking.

3. You should get no more than a 100-rpm increase when going to idle cutoff. Any more than 50 rpm means that idle mixture is too rich.The optimum idle setting is one that is rich enough to provide a satisfactory acceleration under all conditions and lean enough to prevent spark plug fouling or rough operation. A rise of 25-50 rpm will usually satisfy both conditions.

4. Check the aircraft belly again.