Lycoming Fluctuating Oil Pressure

Below is a description of the problem that is quite lengthy. Sum it up to say that the oil pressure keeps changing. What could it be?


Here is my take on all of this: If you re-label your oil pressure gauge to say "valve opening and closing gauge" then does this better describe what you are seeing on the gauge?  Knowing nothing about your engine but having a good seat for the ball seems obvious.


Engine: IO-360A3B6D
Installation: 1980 Mooney 201 (M20J)

Time since overhaul approximately 100 hrs.
Time since overhaul approximately 24 months.
Always in a hangar.

Approximately 6 months ago I started to notice a gradual decrease of oil pressure once in flight. Start-up pressure was good, take-off pressure was good, initial cruise was good, but shortly thereafter it would start to drop.

Where I would see pressure at approximately 72 psi during flight, it gradually became 70, them some time later during a subsequent flight it would 68.  That eventually became 66, then 64, then low 60’s and even 58.

By the time it hit the low 60’s I began to adjust the oil pressure relief valve and each time I adjusted it, the initial (start-up) pressure re-established itself at around 90 to 94 with cold oil and in flight I would see 3 or 4 psi more than the previous flight.  Generally I would be able to re-establish mid to high 60’s for a flight or two before it would work it way down to the low 60’s.  Eventually I ran out of adjustment of the oil pressure relief valve adjustment screw (spring + ball).

One weird characteristic to mention is that the oil pressure would increase on approach to landing despite the lower power setting on let down to landing. If I had 60 psi at the point of flight just before let down with perhaps 23” HG and 2450 RPM, I would see 64 psi at 15” HG and the resulting RPM.

With the spring adjustment now bottoming out, I changed the spring and ball. I installed the next heavier spring then went flying and experienced exactly the same problem. One thing we noticed was that the oil pressure relief seat didn’t appear to be round thus not allowing the ball to seat properly.  I will be getting the tool to have this corrected.  If I read your oil pressure problem check list it mentions that if this problem is experienced then it is likely that the result would be low pressure at low rpm. It is possible that this could affect the pressure at cruise as well or just by virtue of the way the ball naturally moves away from the seat at higher rpm means that this can’t be the problem?

Other items that have been mentioned are the oil cooler (but it was good at the time of the overhaul of the engine, the gauge (I have 2, and a digital E.I. and the ships gauge) and both seem to give the same readings.  Both are hooked up to the same oil line. Someone else mentioned the veratherm (spelling?), the oil pressure filter by-pass valve, the oil quantity which I usually keep at 6 because I fing that at 7 it blows it back to 6.

By the way, unless I have a bad oil temp gauge the oil temps are normal (mid-green).

If you have any ideas it would greatly appreciated.