The International Cessna^® 170 Association

My oil pressure has been reading at the top of the gauge which has a 50 psi upper limit. We changed the oil, no change in pressure. I dug through the engine log and found a note: "Oil pressure gauge checked for accuracy - found reading 8 to 10 psig high above 2000 rpm. Owner opted to use as is." That's fine, but at the top of the scale I don't know if the oil pressure is really 40 or 50+. The good news is that it reads zero with no pressure. So, which is the most likely source of the failure? The pressure sensor, the gauge, or somewhere in between? The oil pressure line was replaced about a year after the note (four years ago) so I'm assuming that it's OK. I think.

Just as a data point, when my O-300A was overhauled in 2006 a new oil pressure relief valve spring was installed, and cruise oil pressure has indicated 55 psi ever since (approx. 550 hours). I brought it up here on the forums, and no one seems to think it is a problem. As far as I know my gauge is original and ranges from 0-60 psi. If your gauge tops out at 50, maybe you have a newer spring and just can’t read the actual pressure?

There is no pressure sensor. Oil from the engine runs through a restricted fitting, up into the gauge and pushes on a diaphragm in the gauge. Of course someone will argue the air in the line in compressed by the oil and the oil never touches the diaphragm but that is not the point.

Cool! One less thing to check. I think the thing to do is get a new gauge and see what happens. For an aircraft instrument it seems pretty reasonable. Getting the old one out and the new one in... that's a different story.

Thanks!

You really don't know what you have unless you check the calibration with a known accurate gage or a dead weight tester. I checked mine with a calibrated test gauge, from 10 to 30 psi it was accurate, then at 35 it read 36, 40 read 43, 45 read 50 and 50 read 55. I replaced gauge.

I'm hoping that's what the A&P did years ago to determine that the gauge was reading high. I'll talk it over with my local A&P today and see what he says. I hope he as an accurate pressure meter that he can attach to the pressure line and see what's what.

Hook up your oil pressure gauge to an air compressor line with an adjustable pressure regulator to calibrate the oil pressure gauge.

It was almost 20 years ago that I visited with TCM about the 55 psi my engine indicated (and that I’d confirmed correct) and was assured that the old spec about oil pressure was based upon the original C145 which had different hyd lifters. 55 psi is fine for engines overhauled since approx 1960’s. <edit> (excessive oil pressure would cause early hyd lifters to “float” was the explanation given.)

I used my differential pressure tester to check my oil pressure gauge. You can run the pressure up and down in 5 or 10 psi increments and see how close your gauge is to the tester gauges going both directions. As long as there is no leakage, all three gauges should read the same when calibrated.