Oil pressure gauge advice

[BritishCubBloke]

More or less since I've owned my aircraft (65 hours of flying time), the oil pressure gauge has not been entirely consistent. (It's a 150HP Lycoming O-320 A2B on a PA 18.) Sometimes I'll start up and do the run-up with the oil pressure near the top of the green. Then, it might move to the upper orange for the take-off and 10 minutes afterwards (at 2250rpm) until the oil warms up and it comes down to the upper green. Why is this? Why will it sometimes start and run in the green and sometimes in the orange? Sometimes it will start in the green and remain in the green. Sometimes not. It's never very consistent, until a while into the flight. Is this lack of consistency due to:

a) Cold engine, cold oil?
b) Sticking gauge?
c) Malfunctioning oil pressure relief valve?
d) Something else?

All through, I should say, the engine has run sweetly and smoothly and has only ever started first time. It's a beautiful, low-time engine, running 20/50 Multigrade and uses very little of it.

BCB

 

[Dave Calkins]

BCB, No worries.

Lycoming oil pressures for the same crankcase as yours are stated as follows:

95 PSI (that's POUNDS PER SQUARE INCH)- Normal

55 PSI- Minimun

25 PSI- Idle

115 PSI- Start-up, Warm-up, Taxi, and Take-off.


Oil pump output volume is predicated by engine RPM.

Full pump output volume at Take-off power with cold (relative) oil cannot be dumped by the relief valve.

Therefore, the higher than comfortable (for the pilot) oil pressure indication with cold (relative) oil. And also the decreased pressure indication after oil is warm.

This is all normal.

If you were actually having an over-pressure problem, seal failures would be the first evidence. This engines seals are, of course, overdesigned for their purposes, but the most likely place would be somewhere that receives high-pressure oil. The oil screen housing gasket would be a good place to start. I have seen the forward mounted oil coolers on the PA-18 bloated by congealed oil in the winter and the pressure build-up behind them.

I hope this helps your nerves.

By-the-way...unless your gauges have been calibrated recently, never trust them. Tachometers are notoriously inaccurate. Cub ASI's also. But your oil presure instrument may be just fine.

Dave Calkins.

 

[BritishCubBloke]

Thanks a lot, Dave. Much appreciated. I'm going to keep a log of what happens. I strongly suspect you're right and that it's nothing to worry about. I'll let you know.

BCB