How to Monitor Your Engine’s Condition

With a few extra simple checks before, during, and after each flight, you can gain a broader picture of your engine's health, and increase your confidence in your aircraft.



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

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

  3. Smell inside the engine compartment for any fuel smells. Smallfuel leaks evaporate fuel as they leak and may not be enough todrip. Leaks may occur at primer fittings, hose connections, orthe hose itself. One sniff in the engine compartment and you’vechecked all of the fuel connections for leaks.

  4. Check the color of the oil on the dipstick. If it looks likeblack lacquer then the piston rings are leaking combustion gasinto the oil.


  1. Listen for any out of the ordinary noises as the starter turnsyour engine. You should hear the starter, the clanking of theimpulse couplings, and no wheezing of air out the engine breatheror intake.

  2. On your Continental 6-cylinder engine, does the propellerturn with the starter? If the starter turns but the propellersometimes doesn’t, then the starter adapter is slipping and needsto be repaired.

  3. On Lycoming engines if the starter turns but the propellerdoesn’t then the starter Bendix is starting to stick. Usuallycleaning and silicone spraying the starter Bendix shaft fixesthe problem.

  4. Does the engine kickback when starting? If it does, then youhave 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 probablyneed repair.


  1. Many engine problems are first noticed during idle. Engineroughness, caused by carbon fouled spark plugs, lead fouled sparkplugs, a sticky valve, or a hydraulic lifter not operating properlyare 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 notfiring constantly or that the engine is operating at a too richfuel/air mixture. Lead fouled spark plugs indicate a rich fuelmixture or that the power is being increased too rapidly at takeoff.

  3. Bad hydraulic lifters are more noticeable during idle thenduring flight. A worn hydraulic lifter that leaks oil causes rocker arm to valve clearance. The rocker arm strikes the valvetip instead of pushing the valve open, resulting in a tappingnoise. The noise goes away as the cold engine oil flows into thehydraulic lifter. Cold oil, being more viscous, doesn’t leak outthe hydraulic lifter as fast as hot oil. This causes the hydrauliclifter to pump up, closing the tappet clearance and causing thetapping noise to go away. This is fine and should not be a concernif the noise goes away shortly. If tappet noise occurs regularlythen replace the hydraulic lifters. Worn or defective lifterscause the valve to pound against the seat, possibly causing valvebreakage.

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


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

  2. If takeoff rpm is low on a constant speed engine then theproblem may be in the governor and not in the engine. Check tosee if you can reach redline rpm in cruise flight. If a constantspeed propeller airplane won’t reach redline rpm in cruise, thenthe 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 linebecause of the low engine loading.

  4. Monitor for engine smoothness and power.

  5. Is vacuum pump pressure normal? As the vacuum pump startsto fail it often produces lower suction for a flight or two beforefailure.


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

  2. The higher the altitude the less resistance to arching withinthe magneto. Therefore, a marginal magneto often causes slightengine roughness during the climb, only to clear up when you reducepower or descend to a lower altitude.

  3. To some degree oil pressure follows oil temperature and oiltemperature follows cylinder head temperature. As oil temperaturegoes up, oil pressure goes down. As cylinder head temperaturegoes up, oil pressure goes up. This can be used as a crosscheckof proper gauge operation. The relationship is not linear andsometimes may not exist. For example, increased heat transferfrom the cylinders to the oil occurs when the piston rings startleaking hot combustion gas into the oil. This causes oil temperaturesto rise without a corresponding increase in CHT temperature.


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

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

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

  4. Check the aircraft belly again.