Oil pressure variation

KRyan

KRyan

Pre-takeoff checklist
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KRyan
I have a customer who is experiencing engine oil pressure fluctuation between 68-75 psi hot (205 F) and always has. Motor has 100 hours time. Is this normal? If not, what might be the cause?
 
KRyan said:
I have a customer who is experiencing engine oil pressure fluctuation between 68-75 psi hot (205 F) and always has. Motor has 100 hours time. Is this normal? If not, what might be the cause?
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Bad oil pump, bad regulator, or an air leak in the oil pick up.

what engine ?
 
Tom-D said:
What would make it fluctuate?
Once pressurized and the pressure remains stable why would air in the gauge line fluctuate.?
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It won't, or at least not that I've ever seen. In the test cells at work people are supposed to purge the lines but it never happens. No pulsation seen there.
 
mondtster said:
It won't, or at least not that I've ever seen. In the test cells at work people are supposed to purge the lines but it never happens. No pulsation seen there.
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right
 
He believes that this variation is causing his prop to hunt. I think that it could be the cause, but since the governor is basically a booster pump, the fluctuation from the engine oil pump would be more or less "invisible" to the prop, but I could be wrong.
 
KRyan said:
He believes that this variation is causing his prop to hunt. I think that it could be the cause, but since the governor is basically a booster pump, the fluctuation from the engine oil pump would be more or less "invisible" to the prop, but I could be wrong.
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I'd start by sending in the prop governor to be checked or overhauled. With a hunting prop it could be that the governor is redirecting oil to the prop or back to the oil pan causing the variations in oil pressure.

Did the prop and governor get looked at during the overhaul? What was the cause for the overhaul?
 
He says the prop and governor are new. What I really need to know is a 7 psi pressure variation "normal." If it is, then he has to look elsewhere, if it isn't, fixing his oil pressure problem may solve his prop hunting. It's a homebuilt, so he could have all kinds of issues.
 
He's in Melbourne, Australia. Tom's not going . . .
 
If it's any help, he's got a Continental IO-320.
 
Btw....the GOV is not a pump but a needle valve that opens and closes to regulate the prop pitch as a function of RPM and oil pressure.
 
KRyan said:
He's in Melbourne, Australia. Tom's not going . . .
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Bet me.. pay the way I'm there :)
Checkout_my_Six said:
Btw....the GOV is not a pump but a needle valve that opens and closes to regulate the prop pitch as a function of RPM and oil pressure.
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Many prop governors are pumps, he doesn't tell us what prop and system he has. but if he has the simplest governors he may have sticking slide valve. new governors do that.

OBTW they are slide valves, not needle valves. they only have 3 positions, on speed, under speed and on speed.
 
KRyan said:
He says the prop and governor are new. What I really need to know is a 7 psi pressure variation "normal." If it is, then he has to look elsewhere, if it isn't, fixing his oil pressure problem may solve his prop hunting. It's a homebuilt, so he could have all kinds of issues.
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variations are not normal, home built or not.
 
That's what I needed to know. I'm going to tell him to fix the pressure problem, and perhaps that will fix his hunting problem as well.
 
KRyan said:
He says the prop and governor are new. What I really need to know is a 7 psi pressure variation "normal." If it is, then he has to look elsewhere, if it isn't, fixing his oil pressure problem may solve his prop hunting. It's a homebuilt, so he could have all kinds of issues.
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In that case, I'd maybe try a different gauge first to see how it reacts. I've flown some planes that you can see a slight wiggle in the oil pressure needle, but I've never seen a 7 psi swing. I've always assumed the slight needle wiggle was due to oil pump pulsation and possibly the lack of a snubber in the oil line.

I'm not too familiar with the Continental IO-360, so specifics on that engine will have to come from someone else.

One additional thought is the possibility of a sticking pressure relief valve or maybe a crack in the oil pump pickup tube like Tom mentioned.
 
FWIW a prop hunting can cause oil pressure variations if it is not quite getting enough oil to the prop. The oil transfer from the governor is contained so it goes to the prop by pieces of nylon set in grooves inside the case.
 
Tom-D said:
Bet me.. pay the way I'm there :)

Many prop governors are pumps, he doesn't tell us what prop and system he has. but if he has the simplest governors he may have sticking slide valve. new governors do that.

OBTW they are slide valves, not needle valves. they only have 3 positions, on speed, under speed and on speed.
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You talk like an expert at keeping engines running. :)
 
I frankly don't know prop servos but I do know industrial servohydraulics. So -----

The engine oil pump relief valve in Lycomings (and Continentals) is a very primitive but reliable affair consisting of a direct acting spring on a ball or sleeve. It's ability to maintain a constant pressure is pretty limited if there is a substantial variation in downstream flow restriction or demand. If the oil temp is over 200 degF and the engine a little worn, I'd guess that that the oil pump bypass relief valve is operating nearly if not completely closed. There then could be a prop servo control supply pressure fluctuation reflected from the prop control flow demand. I think the prop control would be a closed center three way hydraulic valve so it has a varying demand. There is a hydraulic pressure boosting pump and I assume a second pressure relief valve. Does that 2nd valve dump excess flow to the crankcase? I assume so.

A key test would be to see if the instability onset is highly oil temp sensitive. And what is the approximate frequency/period of the fluctuation?

or maybe just disregard this please................
 
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Engine oil pressure is a product of oil viscosity, engine internal clearances, oil pump, and oil pressure relief valve. You have min and max oil pressure reqirements, the pressure relief valve works to maintain these pressures for all operating conditions, engine idle RPM thru Takeoff or max power RPM.

The propeller governor probably has a high pressure oil pump built into it. It boosts engine oil pressure to required amount to change blade angle.

Since the engine oil pressure relief valve is least expensive, I'd make sure it's adjusted and functioning normally.

I suspect the prop governor is malfunctioning and the reason you see oil pressure flux is because the relief valve is working to compensate for it, but being mechanical, there's lag.
 
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