Loss of oil px

[Atlas Shrugged]

I recently experienced a slight loss of oil pressure on a short flight, however there was no accompanying increase in oil temperature. There was also a low suction indication of just under 4" at normal cruise power during the latter part of the flight. All other engine indications were normal. Other than the aircraft consuming more oil than usual during the past 25 hrs of flight time, around 1qt per 3hrs, there were no other problems. A quick finger check of the exhaust pipes on the ground indicated carbon deposits in the rear pipe (presumably from the rear cylinders) in the form of thick dry black residue, which I suspect might be caused by oil getting past the rings or a damaged or broken ring. The front pipe is clean, although the residue is still a little darker than normal. This has been steadily getting worse over time. From my limited knowledge I understand that if the rings are worn it can lead to a build up of crankcase pressure in which case oil will be blown out of the breather, or oil can sometimes be pumped overboard due to a faulty vacuum pump. I don't think that this was the case as after the flight, the oil quantity hadn't changed. Also, a compression test on the engine has since been carried out which failed to reveal any abnormalities so I guess that rules out a broken ring. On a subsequent test flight all indications were normal, apart from a high oil pressure indication during takeoff, which now has me even more confused. The engine is a Lycoming IO-360-C Series with around 1200 hrs on it. The last oil filter change was about 25hrs ago and nothing abnormal was found there either.

Any suggestions?

[barit1]

I'm not sure of the exact number of rings per piston on a IO-360-C but usually there are both compression rings (near the top of the piston) and oil control rings (nearer the bottom).

If a bottom ring is broken it will allow more oil to cling to the cylinder walls on a downstroke. This will not show up on a compression check, but will increase oil loss.

[Irish Steve]

Might be worth checking that there isn't a small piece of debris in the pressure relief valve, which under the seat would cause a possible pressure drop, and if it then got behind the ball, could cause the opposite of high pressure, which would tie in with the symptoms on the pressure side.