Friday, June 18, 2010

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.


  1. Could this problem be in the governor? It is mentioned that the pressure drops during cruse and rebounds on landing. The governor sends oil pressure to the propeller to increase pitch for cruse. If there is a problem in this section could excessive oil flowing back to the sump resulting in the observed pressure drop?

  2. Good idea - I will pass it on.

  3. I'm having exact symptoms with Grumman Yankee with Lycoming O-290-D2 with fixed pitch prop. Good pressure on start, run up, takeoff. After 5 minutes flying (oil temp reaches 185F) oil pressure drops to 60. When I reduce throttle for approach pressure returns to 75. Basically Oil P=60 for RPM>2000, Oil P=75 for 1500<RPM<2000. Temperatur constant ~185F.
    Owned this airplane for 10 years and just started seeing this recently 25 hours after overhaul.
    Helpful suggestions most welcome.

  4. I have the same problem on my o235-c1. I just completed a top overhaul and the pressure is good until I get to curise. It gradually decreases down to 60 psi and will come back and recover at 75psi. Its not consistent. Did you guys figure this issue out??

  5. I have been having the same problem for last several years. O320E2C fixed pitch in an RV-6. Oil pressure is high at start up, then after it reaches 100 deg I can do a run up with out redlining oil pressure. After in flight and 180+ deg the pressure drops to mid 60's. When I pull the power back to below 1400 RPM's the pressure jumps up to mid 80's. Any thing above 1400 RPM's and it drops instantly to mid 60's. Oil analysis has been good so should I be concerned?

  6. Was there a solution to this problem? could it be the oil suction screen (in the sump) being clogged? This screen should be inspected every oil change, or at least on every annual.

  7. I am having similar issues on 2 O-360s. Aircraft sat idle for a few years, ran ok for about 10 hours, and now we see the temp hits 220 degrees, oil pressure drops and the propellers go into feather position. Checking the vermotherm, sent out the oil coolers which were not hot at all, and if the vermotherm and oil coolers check out ok, then what? Maybe prop governors?