oil pressure vs temperature

gerrychuck · #1 12-17-2011, 02:33 PM

Flying today with unseasonably warm (37 F at altitude) temps. I have my oil cooler inlet covered with foil tape, and the oil temp gradually crept up to about 220 and stabilized there; definitely the hottest I've seen so far. As the flight went on (1.2 hrs air time) oil pressure started to slide downward a bit, stabilizing at about 30-35 psi, where it normally sits at about 60. Oil quantity was okay; just under 6 qt before the flight, and probably about the same after (checked hot; probably still draining down, quantity was still over 5.5). All readings for both temp and pressure remained in the green on my gauge, but the pressure was definitely a bit lower than what I normally see.

Is the lower than normal pressure likely related to lower viscosity with the higher temps? Can't think of any other reason, but I'm not sure how much the temp would affect things.

Incidentally, I already have the Nonstop Aviation butterfly controller for the oil cooler sitting on the bench in the hangar waiting to go in; if we keep getting warm weather like this it's going to become a high priority...

Thanks in advance

flyinga · #2 12-17-2011, 02:47 PM

Oil pressure will drop with an increase in oil temp. You are probably looking at a normal indication.

gerrychuck · #3 12-17-2011, 02:51 PM

Thanks Jim; that's what I thought but figured I should check to be sure.

Kyle Boatright · #4 12-17-2011, 04:11 PM

Yes, oil pressure varies somewhat with temperature. Still, at an oil temp of 220F, you shouldn't see pressures as low as 35psi in cruise. IMO, there is a problem somewhere that might impact the longevity of the engine.

As a point of reference, the bottom of the green arc for oil pressure is 60 PSI. It wouldn't cause me heartburn if the reading was 58psi, but 35 psi is well out of my comfort zone. Even when I was fighting oil temperature issues during my fly-off period and saw oil temperatures in teh 230's, the pressure never dropped below 60psi at a cruise power setting.

gerrychuck · #5 12-17-2011, 04:45 PM

Interesting. My gauge is set up to indicate 60 psi as optimal pressure, and that's usually where it sits with normal oil temps. It's a digital gauge with switchable oil temp and pressure digital readout in the center of the gauge with light indicators on either side for left is pressure, right is temp; the lights are green down to 30 psi on the pressure side.

Wayne Gillispie · #6 12-17-2011, 05:12 PM

My IO-540 runs about 75-85 psi in cruise. Min is 55 psi. I left my oil cooler damper closed off too long and got a warning at 220F. Oil pres was stilll above 55 psi. I am running SAE 50 min oil for now. Ambient temp was 30F.

I would check the pressure with a flex line and known accurate dial type gauge. It does not sound like the correct gage if your engine specs are anything like mine.

Alan Carroll · #7 12-17-2011, 05:56 PM

This definitely sounds too low to me. I have the same engine, and at that temperature I'd expect to see no lower than 60-65 psi. If I recall correctly the Lycoming engine manual says the minimum should be 55 psi. Assuming the pressure sensor and gauge are OK, you might need to adjust the oil pressure relief valve.

sailvi767 · #8 12-17-2011, 06:17 PM

Quote:

Originally Posted by gerrychuck

Interesting. My gauge is set up to indicate 60 psi as optimal pressure, and that's usually where it sits with normal oil temps. It's a digital gauge with switchable oil temp and pressure digital readout in the center of the gauge with light indicators on either side for left is pressure, right is temp; the lights are green down to 30 psi on the pressure side.

Gerry, at cruise power most manuals list the minimum oil pressure as 60 PSI. Some models are listed at 55 PSI. This is at cruise power. At lower power settings oil pressure will be lower. 30 is fine for a idle or low power setting. Many set their EMS up to reflect that and not issue a warning until below 30 PSI. That may be why your green lights are good to 30 PSI. I see about a 3 PSI reduction in pressure from oil around 180 degrees to oil at 205 degrees. The drop here seems to be to much. I would double check quantity. A friend had low oil pressure with some fluctuations at cruise. Turned out to be simply low oil. I would put 7 quarts in the engine and see how it does. I know they say most 360 are good to 3 or 4 quarts but his does not like below 5.

George

jrs14855 · #9 12-17-2011, 06:35 PM

Way too low. There are some very knowlegeable people that believe high green-75-80 psi- provides additional (and badly needed)lubrication to valves/guides. If everything is correct in the engine, red line oil temp should cause less than 10 psi drop in oil pressure. I have flown Pitts S2B's that ran close to redline oil temp with no significant drop in pressure. Four cylinder pretty much the same.

gerrychuck · #10 12-17-2011, 11:34 PM

Thanks for all the input. I should mention that the low readings were noted on descent from 6500 to 3000, with modest reduction from cruise power, coupled with the high oil temps. Cowlings will come off tomorrow; I'll install the butterfly for the oil cooler and have a very thorough look for leaks. The manual for my engine does specify 55 psi as the minimum, and I don't think I've seen indications lower than 60 in cruise, and often 70-75. I admit I didn't take a lot of notice; lights were green, and I was busy learning to fly the plane. Didn't really start watching the gauge closely until it got cold and I really started watching the oil temp. I'll double check oil quantity as well and go from there.