How low is too low for oil pressure

[JoeW]

I have a oil pressure question. At full throttle my oil pressure is about 70-75, reduced throttle about 65 w/ temps about 180-190. Today I was doing a lot of Young Eagles Flights, OAT about 85-90. I noticed my oil pressure down to about 62 with temps about 200 to 210. After the flight I checked the oil quantity was 6QT, which is the normal capacity for my O-235. I'm running straight weight Aero Shell W100. Recently did an oil change and had no metal or anything in the filter screen. Should I be concerned?

Thanks, Joe

 

[Fortysix12]

no. What are the green numbers for your airplane? 210 for a supercub is getting up there. Why?

 

[JoeW]

correct numbers

Per the O-235 manuel it said Lowest desired is 65 highest is 85 I believe green arc is 60 -85. Preferred is 75. As far as temps, 180 Optimal 285 max. I wonder if it is a clogged/partial obstruction in pressure relief valve? It was different than other indications, so I became a little concerned. Joe

 

[RON NORMARK]

Oil pressure on O235

My Super Cub with a O235 runs hot . It was 91 degrees today and it got up to 220 degrees. Lycoming reps told me not to worry about it as long as it stays below 240 degrees. I reduce prower when it gets to 220. Makes me feel better.

Ron

 

[Steve Pierce]

Oil viscosity gets thinner the hotter it gets. You can put another washer in the oil pressure relief valve to bump your pressure up a bit. Operated an O-235-C1 for many years in the heat of Texas and this is a normal thing, especially without an oil cooler. On a lot of airplanes the manufacturer put a blast tube on the oil screen/temp probe housing. I guess it drops the temp a bit.

 

[T.J.]

The little red, yellow or green marks on the face of the instrument mean something. It it is in the correct range, your good to go. If the low pressure bothers you, as Steve said, add washers.

 

[Fortysix12]

I missed the part about the engine being a O 235. Now that it's summer I will see my temps at 220 to 230 everyday. The oil pressure does not come out of the green except for idol. This a 12 with no oil cooler. If I run hard the temp will nock on the doors of red line but don't recall the pressure being an issue even at that temp. I will fly it soon and take some notes. Steve cure for another washer on your engine might be all you need. Done a oil analysis lately?

 

[behindpropellers]

Same question with C90 in PA-18-95.

What is normal operating temp/pressure?

I borrowed a friends cub this weekend and it was running aroun 20psi and 210 degrees, OAt was around 80-85 and changing the rpm between 2000 and 2500 would not get the oil temp down.

Tim

 

[RMREBOB]

Steve Peirce said "You can put another washer in the oil pressure relief valve to bump your pressure up a bit."

Steve I must be missing something. My understanding was that adding washers to the oil pressure relief valve would only raise the high end of the pressure as this is the only time the valve opens.

Correct me if I'm wrong.

 

[Steve Pierce]

It increases the pressure needed to unseat the pressure relief ball, thus increasing the oil pressure itself.

 

[RMREBOB]

But only at the high end of the pressure range, At low pressures the valve is seated all the time allowing no oil to bypass. I.E. You can't raise low oil pressure, only limit high oil pressure by increasing the spring tension with added washers.

 

[Cajun Joe]

Good point Bob,
But don't discount oil pressure being lowered by escaping around a badly seated valve which could be caused by a piece of metal being slammed down by the ball. Of course this would also lower the upperend as well. Yet a good check is that seat then go for the mains.

Joe

 

[Ron Babos]

Correct, adding a washer only increases the max pressure. As the oil temp increases, so will it's leak rate and the pressure drops a bit. As long as it is within specs, I'd tend to leave it alone. The washer may put you over pressured on the colder days.
Ron

 

[Steve Pierce]

OK I'm lost. If you raise the tension on the oil pressure relief spring won't it make the lower rpm oil pressure higher than if the spring tension were less. I guess I never relized that the relief valve only unseats at the max pressure. I am going to experiment with this one.

 

[Tom Jones]

Steve,

The relief valve is not part of the pump, as a bleed valve might be, and cannot have an effect on what pressure the pump produces. The pump will move oil and create pressure at a rate purely dependent on RPM (viscosity effects aside). The valve can only limit the maximum pressure in the system by opening at a certain point and relieving the excess. It can have no effect on any other pressure point.

It's a pop-off valve, just like the radiator cap on you car.

Tom

 

[Steve Pierce]

I understand how the system works. My question is at what point does it actually open. The problem here as I read it was that as the oil heated up on an O-235 without an oil cooler the pressure drops. My solution would be to up the pressure a small amount so that when the oil is cold it is in the upper limits of the green and hopefullr still in the green when it is hot. I have had this same problem with the 90 hp SC I fly. When it is 100 degrees here in Texas I am just barely in the green and i Figure if I bump the pressure up it should raise my pressure when the oil is hot.

 

[Aviator]

Agree with above comments. Pressure doesn't exist without resistance. If the relief valve opens at say 100psi and the effective ball area is 1/4 sq in, then the valve spring resists a 25lbs load regardless of oil viscosity. Until that load is exceeded, the valve does nothing. Anytime the oil pressure is 60, 70, 80 or 90, the valve is as closed as at 99psi. By adding a washer, the spring can resist a higher load, say 26lbs or 104psi, but it won't make any difference because the pressure loss is caused by internal system leakage (bearings, etc), not by the opening of the relief valve. Adding a washer is means to compensate for a weak valve spring. My $.02

 

[Cimarron]

I don't think the pressure valve is just at max a relief valve. The ball is always off the seat, the spring tension changes the amount of oil that will bypass at what pressure. I dont think the valve completely closes till about 35 PSI so in the whole opperating range boosting the spring tension will boost the pressure.

 

[Steve Pierce]

My thought is that it is kinda like a voltage regulator opening and closing all the time to regulate the pressure.

 

[Aviator]

I don't think the pressure valve is just at max a relief valve. The ball is always off the seat, the spring tension changes the amount of oil that will bypass at what pressure. I dont think the valve completely closes till about 35 PSI so in the whole opperating range boosting the spring tension will boost the pressure.

Interesting point. If the valve varies the amount of oil bypassed (between min and max pressure), then it's not a relief valve but a pressure regulator (my O-320 has one of those).

 

[Ron Babos]

The pressure relief valve is designed to relieve excess pressure at a given maximum setting. Ok fine , it may be partially open slightly below that, but that means nothing to us. It depends on oil viscosity and engine rpm. If an engine is totally baffed in the bearing area, you can put 20 washers in there and the pressure will only get to the point where the leak rate is in excess ot the pumps flow, and you will never get any higher. This is with hot oil of course. (cold it may be excessive) At this point the pressure relief valve is closed, and is set at a value that cannot be reached. A rebuild would be good at this point.

 

[Cajun Joe]

You guys.

Joe's original Question (Remember Joe W??) was "should I be concerned?"
Would someone please tell the gentleman (because I have no credentials under the hood) that since he was flying the young eaglets around he was obviously at max power then a bit of a slow flight then a taxi and shut down - load another eaglet- start up then a max power climb etc....... Joe told you he was pushing high temps and well he should with the engine getting no time for a nice 25 minute 2400 rpm cool flight so his water thin oil clocked in at 7 pouinds below a normal pressure.

Now, "SHOLD HE BE CONCERNED."

No, Joe, have a Meyers and Tonic, put on a little Buffett -- "questions that bothered him so" -- and let the thing cool down on the ramp. Tomorrow the pressure will be fine.

 

[Steve Pierce]

Oil viscosity gets thinner the hotter it gets.

If the pressure makes you nervous put another washer under the spring. It will bring the pressure up. If it is in the green I would leave it alone.

 

[T.J.]

As above, see below.
I answered the question.
If its in the green, good to go.

 

[jcrowles]

I have a pa-20 that I have owned about 40 plus years. Many years ago I had the engine overhauled and for the next 7 - 8 months I had the same problem with high oil temps and oil pressure in cruise at 2450 rpm , at 45 lbs. I tried the washer trick and it helped to a point. I finally pulled the ball on the pressure relief and found a small piece of carbon on the ball! I cleaned it up and re-installed it and the pressure went up to 80 lbs. For several months I had to clean it off nearly every time I flew the plane. Eventially it straightened out . I quizzed the mechanic and found out he didn't pressure clean the case at overhaul!