Piston engine troubleshooting

Piston Engine Troubleshooting By Thomas Ehresman March 1998 You work in a shop that specializes in turbine equipment. The few cabin class piston twins for which the FBO is responsible get their maintenance done elsewhere. From time to...

In the winter the problem may be nothing more than a congealed oil cooler. The oil will get thick enough when bitterly cold to obstruct flow through the cooler. This will cause the oil temperature to rise and the oil pressure to fall. When a congealed oil cooler does free up a near instantaneous rise in oil pressure and fall in oil temperature will be noted. The fix here is the proper grade engine oil for ambient temperatures (consult the engine operators manual) and winter baffling for the oil cooler or possibly the whole engine.

Excessive piston ring blowby will also elevate oil temperatures because of the hot gases escaping into the crankcase. See the section on low cylinder compression for tips to troubleshoot this source.

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5. Low oil pressure:
The three most important factors that determine oil pressure in any given engine are: oil pump volume, engine internal clearances/leakage, and oil viscosity. The two most common causes of this symptom are oil viscosity and internal clearances/leakage.

Oil viscosity is affected by oil grade, temperature and oil damage. The wrong oil grade for ambient temperatures or high oil temperatures will often cause low oil pressure. Oil can be damaged by excessive heat and/or excessive operating time before oil change. All these factors will tend to thin the oil and cause a pressure drop.

Any excessive internal leakage in the engine will also cause a drop in pressure even at normal operating temperatures. The most common culprit here is debris under the oil pressure relief valve. This acts to hold the valve off it's seat and bleed off oil, dropping the pressure. Remove the valve and clean the face and seat. Note the type of debris that was caught in the valve. This may be an indication of larger problems as noted in the next section.

Excessive clearances from worn bearings, worn valve lifter bores, worn oil pump gears, bad prop oil transfer collars, or missing/loose internal oil passage plugs (just back from the engine shop) will cause a drop in oil pressure. An oil analysis and a look at the oil filter will usually cue you to internal engine problems from worn parts as the cause of this symptom. Oil pump volume is not something that we can change in the field usually, but is an aggravating factor on some engines. Engines with standard volume oil pumps but high flow demands are very sensitive to viscosity and clearance/leakage factors. This is most common on naturally aspirated engines that have been turbo-normalized by someone other than the engine manufacturer. If a higher volume oil pump is not a part of the package, the oil pump may be hard pressed to deliver normal pressure when the viscosity gets lower (higher temperatures, aggravated by hot turbochargers).

Some aircraft have flow restrictor orifices in the turbo supply line just to keep the oil pressure up. In these engines, any change in flow demand, clearance, or leakage is directly reflected in the oil pressure when at normal operating temperature.

6. Low cylinder compression:
This problem is usually found during a scheduled engine inspection. However, excessively low compression on one cylinder is usually noticeable when running the engine as well.

The three most common leakage points are intake valve, exhaust valve, and rings.

If an intake or exhaust valve is leaking it will be plainly audible in the intake (intake valve) or exhaust (exhaust valve) system. If the leakage is not bad the valve may be relapped without pulling the cylinder.


The valve may also be sticky and just require some cleaning of the guide to devolve deposits. This can be found by checking the looseness of the valve in the guide when the valve rocker cover is removed.

Worn or sticky rings will cause a loss of compression and power also. This is called "blowby." There is always a certain amount of blowby present in every engine. The best way to determine if blowby is excessive is to make sure the intake and exhaust valves aren't leaking when the compression test is done. Remove the oil fill cap and listen for escaping air past the rings. Continental allows compressions much lower than Lycoming but only under special conditions, namely no valve leakage and using a special compression tester.

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