The Cessna 120-140 Association Forum

Ever since the overhaul on my C85-F about 30 hours back I have a recurring problem with what seems to be a sticking oil pressure relief valve. My normal operating oil pressure is fine, well within all limits, startup and cruise. On several occasions however, I have experienced low pressure, about 30 in cruise, 10 on descent, ZERO at idle. The solution is to remove the pressure relief valve acorn nut and piston, clean the valve seats and reassemble (nearly impossible with my old fingers!) After this procedure on next startup the pressure is right up in the 30 to 60 range, slightly lower after warmup. That is what I expect as normal.

These episodes come on suddenly, and seemingly randomly. I have already replaced the spring with a brand new one. The valve faces were lapped at the overhaul. At first I thought there must be some tiny pieces of debris blocking complete valve closure, but it keeps happening. Now wondering if there is a burr or something on the piston.

At my wits end, looking for suggestions. Anyone else experienced anything like this?

It could be something blocking the line or fitting going to the pressure gauge. I have unhooked the line at the gage, started the engine let the oil dribble in to a paper cup. When the oil starts looking like that in the oil sump hook the like back up to the gauge and see what happens.

Enough time has gone by that I feel reasonably confident in presenting a follow-up post regarding my earlier oil pressure problems.

In desperation I ordered a used plunger, Part Number 21114, from - you guessed it - eBay. When it arrived I discovered that it was significantly longer than the original plunger. I installed it with the standard (new) spring and my oil pressure has been steady in the 50 -60 psi range ever since.

I believe that the original shorter version was prone to sticking open, and at some point Continental changed to the longer design to address this problem, without changing the part number.

Thanks for the info.