The Cessna 120-140 Association Forum

I’ve got a 140 with a C-85-12. My oil pressure usually sits between 30-34 PSI at cruise (23/2400RPM) but never below. My oil temp usually hovers between 170-180 degrees C. This is at the lower end of the allowable range according to the overhaul manual (30-60PSI). I have already added washers to the relief valve. Any thoughts on cause/how to correct?

http://www.bowersflybaby.com/tech/fenton.html

Harry Fenton has quite a few threads on oil pressure and temps. He is not a fan of the washer trick for the pressure relief valve. I'd start by reading up on his collection. Go to the site above and scroll down to the index that has different subjects, you'll see a bunch on oil pressure for small Continentals.

When I rebuilt my 0-200 the oil pressure was originally fine at 45 in cruise. The overhaul shop said the accessory case needed the oil pump gears re-bushed and the gears replaced. I am over 55psi now which kinda gets my attention but it is in spec. I paid particular attention to the pressure release plunger on clean up and where it seats. New spring. Fresno Airparts has a new spring and plunger for about $30 total.

Could also try straight weight oil like Aeroshell W80 or W100 for a look see.

switching to the new spring significantly increased my oil pressure especially with oil temp below 180. Once the temp is 180+ the pressure is in the mid 30's now. 1800 SMOH.

Do you happen to have the part number for that spring?

tonycondon wrote: ↑Sun Apr 13, 2025 4:56 pm switching to the new spring significantly increased my oil pressure especially with oil temp below 180. Once the temp is 180+ the pressure is in the mid 30's now. 1800 SMOH.

https://www.fresnoairpartsco.com/produc ... re-relief/
https://www.fresnoairpartsco.com/produc ... per-crush/
https://www.fresnoairpartsco.com/produc ... re-relief/

And while you are at it you should have your own copy of the parts manual, here's a PDF to download

http://rob.com/matt/manuals/O-200_Parts_manual.pdf

Get a calibrated oil pressure gauge on it before you do anything else. If it's over 30, and at least 10 at idle, you're good to go.

There are testing limits for the spring in the overhaul manual.

As far as I know, the "washer trick for the pressure relief valve" is not approved, i.e. prohibited.

harejac wrote: ↑Sun Apr 13, 2025 1:23 pm I’ve got a 140 with a C-85-12. My oil pressure usually sits between 30-34 PSI at cruise (23/2400RPM) but never below. My oil temp usually hovers between 170-180 degrees C. This is at the lower end of the allowable range according to the overhaul manual (30-60PSI). I have already added washers to the relief valve. Any thoughts on cause/how to correct?

I think that putting washers under the spring just changes the opening pressure point - once the relief valve opens, the pressure will be whatever the pump can provide juxtaposed with what the bearing clearances demand.

Sometimes the seat where the relief valve lands can become scored or otherwise non-conforming with the profile of the valve face. Some mechanics will lap the seat in situ which can help.

What is your start up oil pressure when the engine is cold?

I don’t understand why you want to fix something that is not broken.
It is in the green and good to go.
I would be willing to bet that Continental built in a margin of safety when they decided to make 30 psi the bottom of the green.
Keep an eye on it and if it goes below the green then do something about it.

Just my two cents.

Rick