Low oil pressure

[Spirit of Pappy]

I purchased a not completed Starduster Too project that came with a Continental 0-300 with about 270 hours since overhaul. It sat for many years until I got it running. The oil pressure is good at about 60 psi when the oil is cold but drops to 19 psi at full power and oil temp is above 150 degrees. I'm using Phillips 100 50 weight oil. The oil pressure relief spring and plunger checks ok, the pressure sender/gauge checks out as accurate.
Thoughts?
Thanks
Don

[GAHorn]

Hello, Don….and WELCOME!

(but So Sorry for This to be your First Post!)

Thoughts…? Well, unfortunately, it’s not encouraging. Typically that means things are expanding with heat and clearances are opening Up…. usually Main Bearings are too large or worn-out.

It is low-time since “overhaul”…? Was it truly “overhauled” per definition…? or was it a “field” repair and simply “called overhaul”…? Do you know who did it and if they can be contacted? Were Part Numbers listed in the record (logbook)?

I’m not certain what you meant by “the oil pressure spring and plunger checks ok” . Did you measure the spring tension under specified compression/length? Does the spring have a color..? (blue or purple?) Have you checked the suction and pressure-screens..?

If the rebuilder or the records can confirm new parts of the appropriate size for the crank and cam were installed, the next thing that popped into my small mind would be the oil pump itself, which can be accessed for repair/replacement without engine disassemblhy.

Just thinkin’ out loud….

[Bruce Fenstermacher]

George hit all the points.

Small Continental engines which includes the C-145/0-300 have a pressure relief valve which is often confused with Lycoming engines which have a regulating valve. The Continental valve is a high pressure relief valve to bypass high pressure. It is not capable of raising low oil pressure. If all is correct, in operation at "normal" pressure, the valve is completely closed. All you can check with the valve is that the valve plunger is not worn and it' set has no blocking particles stuck to it. The pressure on the plunger is set by the spring and it only sets the high pressure number that bypass would open at.

It is likely your bypass is set at 60 psi which is why at cold thick oil temps you see a high limit of 60. This would be a normal bypass pressure. At any pressure under that the bypass is closed and lower pressure is seen because as the oil warms its viscosity changes to its thinner rated viscosity. As it thins it also more easily escapes between main and rod bearings into the oil sump. Of course the same thing is happening in your oil pump. The oil pressure of the pump is reduced as the oil thins and seeps around the pump gears rather than be pushed forward by them.

Changing your oil viscosity to a thicker higher viscosity does very little to improve warm/hot oil pressure. The problem is as George stated. bearing clearances.

[cessnut]

Agreed. Most likely bearing clearances are too large. For fun, though, you could remove the accessory case and inspect the oil pump like George suggested. The oil pump gears run in nothing but the magnesium casting of the case. I've seen those cases chewed up really bad. It would also be interesting to see what multiviscosity oil would do.

[Spirit of Pappy]

Gentalmen, thank you for your promt replies. I should have included more information in my first post. I did inspect the relief valve components. I tested the spring which I could not determine its color and its force at a specified compression was within limits. I lapped the plunger seat to insure a good seal and confimerd the plunger was not binding. Intersting though there were six washers under the spring which tells me this problem has been manifesting for some time. I also relplaced the oil pump to no avail.
The engine log states it was overhauled about 40 years ago with part numbers and dought the mechanic is still around to question.
I suspect Geoge is correct that bearing clearnances are opened up so I am in the process to dismantle her to see whats goining on.
I will report back what I find.

Thanks again for the helpfull responses.

Don

[GAHorn]

Since you are disassembling it…you will likely have the opportunity to inspect for yet-another possibility….. A main bearing support Crack!

In my 50+ years of abusing these things…. I did run into one engine that was running Perfectly….but had low oil pressure. The “bed” or “support” for the middle crank journal was cracked and opening up, dumping the oil pressure. That engine was suspected of an older “prop strike” …which was treated to a “dial indicator on prop-flange for runout”…and signed-off as good and returned to service. It was sold out of a salvage yard and installed on an unsuspecting new-owner…. who was somewhat disappointed, as you might guess.

Note to All: This is why any prop-strike that results in the need to remove the prop for repair or replacement requires a Compete Engine Disassembly and Inspection (including rotating accessories) per TCM SB 96-11B.*

*
“ A propeller strike is: (1) any incident, whether or not the engine is operating, that requires repair
to the propeller other than minor dressing of the blades….” (emphasis mine)

[Spirit of Pappy]

Well, GAHORN guessed it. After dissaasmby I found the 3rd main bearing had spun and cracked the case surounding the main bearing support structure.
Glad I took the engine out of service to investigate. Who knows how much longer until complete failure!

Thanks to all who replied.

Don