Oil pressure fluctuates with RPM

bsacks05 · #1 05-23-2014, 10:33 AM

My O320, 160hp, carbed engine has always indicated an oil pressure of 65psi while running. This morning on downwind and 1500rpm I noticed the pressure in the 40's and on final near idle it dropped to the 20's. I cleared the active and apply near full power with brakes and the pressure came back up to the 60's. I haven't had time to look at it yet.

In the last month I changed the AN fitting from the engine to a steel one because the al. one cracked after 830 hours. (Don't ask why I used an al. fitting there, I don't know myself. That was 8 years ago.) It has flown 8 hours since then with a steady 65psi until today.

Any ideas before I tear into it this weekend?

jrs14855 · #2 05-23-2014, 04:00 PM

Steam gauge or electrical?? If electric make sure the sender is properly grounded.
The next step would be to remove the oil pressure relief valve housing and clean the ball and seat. if the ball is not perfect, replace it. Next check the oil screen/filter and remove and check the lower screen.
You may also want to do a compression check and oil analysis.
Google Bill Marvel oil pressure, this will take you to a LOT of information on Lycoming oil pressure.
I am a firm believer in high green oil pressure for the Lycomings. I would not even consider flying with 65# in normal cruise.

sailvi767 · #3 05-23-2014, 07:45 PM

I am not even remotely a expert on the engines but your current readings seem in line with what I would expect to see. The constant 65 psi seems like the aberration in readings. I don't recall flying a plane where oil pressure did not track RPM to some degree and 25 psi is allowed at idle.

George

Mel · #4 05-23-2014, 08:29 PM

Your readings are perfectly normal. Please don't "tear into" anything.
Perhaps you are mis-remembering. You should not see 65 psi at idle after the engine is warm.

rhill · #5 05-23-2014, 09:39 PM

Not too many remove the sump screen and clean it every oil change,most engines are returned to Lycoming at and past TBO with the paint still untouched on and around the screen cap.This may not change anything but its an important maintenance item(as per Lycoming).May I ask what oil are you using? Have you added Cam guard?
RHill

flyingriki · #6 05-24-2014, 07:38 AM

Mel,
Is there a need for a metered orifice in the fitting off the engine to avoid fluctuations?
Thanks!

Mel · #7 05-24-2014, 08:39 AM

Actually the orifice is there primarily to minimize oil loss in case of a gauge/sender failure. You should not need it to control fluctuations.

bsacks05 · #8 05-24-2014, 12:59 PM

I never noticed the pressure fluctuating with rpm before. It does not spike or jump like a bad ground. Normal? Ok. I will still look at the oil pressure control valve for debris. No, I did not mean I was gonna tear down the engine. Just meant to say I was gonna remove the cowl and have a look around. Also, I remove and clean the finger screen every oil change.

jrs14855 · #9 05-24-2014, 01:03 PM

Mel-
If 65# is ok maybe you can explain why all Lycoming powered Cessnas are running 90# in cruise??
Have you ever read the Deakin/Scott articles??

DanBaier · #10 05-24-2014, 01:11 PM

Quote:

I never noticed the pressure fluctuating with rpm before. It does not spike or jump like a bad ground. Normal?

If oil pressure relief valve is working correctly, then it does not jump around at that's normal.

I think an earlier poster suggested you take a look at that. Once I was sure of the reading (no issues with gauge, etc.) then that's probably what I'd check next.

Dan