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High oil consumption in Lycoming or Continental Aircraft Engine   

Piston showing oil surfaceTop of piston shouldn't look wet! Too much oil in the combustion chamber.

If exceeding maximum oil consumption limit don't fly the airplane. Why? Oil in the combustion chamber may lower the fuel octane enough to create detonation. Maximum oil consumption limits are in the engine manufacturer's operating manual. Operators of high compression engines should be especially careful about operating with high levels of oil consumption. A industry rule-of-thumb maximum oil consumption is 1 quart per hour. One quart per hour, although easy to remember, is flawed since it does not take into consideration differences in detonation characteristics of various engines. One quart per hour on a six cylinder low compression engine is one thing but one quart per hour on a 10:1 compression Lycoming four cylinder engine is far in excess. Engines modified for high compression such as high compression pistons in the Lycoming O235 should not be allowed to operate with excessive oil consumption. We do not know what is excessive on this modified engine, ask the STC holder.

1. Improper grade of oil.
2. Failure of new rings to seat properly.
3. Failed or failing bearings.
4. Worn piston rings and cylinder barrels.

Lycoming cylinder bore polishing

Bore polishing

White piece of paper is placed into barrel to best view barrel. When the barrel polishes you loose oil control.

A small, barely visible film of rust forms on the cylinder wall. When you start the engine the piston rings remove this small layer of rust. A small amount of barrel is rubbed off - just as if you were polishing the surface.

Repeat the cycle over 10 years and you have a polished bore and high oil consumption. The cylinder wall is still within dimensional limits - the surface, however, is no longer capable of maintaining oil control


5. Worn valve guides.
6. Excessive oil leaks. A small oil leak looks like a great deal of oil.
7. Oil siphoning from engine in flight. Insure that oil filter cap is on tight and the oil access door closes properly. Be sure the breather hose is cut properly and located so there's no chance of siphoning oil from the engine. Continental IO-360 and TSIO-360 engines built prior to 1978, install modified breather per Continental Service Bulletin M80-18 or latest edition.
8. Expander in oil control ring plugged.
9. Plugged injector nozzle. The lack of fuel and combustion pressure allows the oil to bypass the piston rings, thereby giving a false impression of ring problems.
10. Excessive oil out engine breather. This can be caused by blowby of combustion gas past the ring belt. The increased air flow through the crankcase carries more oil out the breather. Continental O360 and TSIO360 engines built prior top 1978 see Continental Service Bulletin M80-18 for breather modification.

Continental pistonNormal tan deposits indicate that this piston operated at normal temperatures. If this piston was overheated it would be black instead of tan.

Normal heat flow is from the piston dome to the piston rings and to the atmosphere through the cylinder barrel fins. If combustion gas is passing by the piston rings then the piston cannot dissipate the large amount of heat absorbed by the piston dome. The piston heats up causing the oil in the ring grooves to carburize causing the piston rings to stick. This causes even more blow-by. In order to evaluate engine problems it is necessary to know what "normal" looks like.

 

 

 
 

 

 

 

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.