Loss of Oil pressure

[Gmanic]

Gentlemen, I am at a loss of ideas here, any suggestion highly appreciated.

Wanted to fix that slow oil leak coming from the forward drain plug and the oil pan to case gasket.
Had the pan off and back on with a new gasket.
It normally takes less than ten seconds for the oil pressure to come up into the 'green'.
So it did during the first start and test flight was normal. Then another CFI flew the aircraft
with a student. He was just going to solo the student for the first time when the student
noticed a loss of oil pressure on the gauge. CFI turned the engine off and made an emergency
landing.
After letting the engine cool down we started it up again and oil pressure would not come up over the minimum red line.
Today, we plugged in a different oil pressure gauge with the same result -no oil pressure.
Right at the case the line leading to the oil pressure gauge line shows a drop of oil coming out when turning the prop through,
but for sure not under 30 psi pressure when cranking. Put compressed air on top of the oil filler to see if that makes a difference and
it does not. Removed oil screen and filter to check for metal shavings suspecting a busted oil pump. Nothing, just clean oil.
Motor has about 550 h on it and has been running flawless up to this point. Pressure relief valve seems normal also.
Our next thought was to pull the engine and open the accessory case to check on the oil pump itself.
Any other thought before we do this?
Please help.

[KS170A]

Are you saying that you are certain that you have at least 30 psi when you're cranking? I would double check that your gauge is ok before you start tearing into the engine.

I would verify your indication system by connecting a known good line and gauge directly to the fitting on the case. While maneuvering around changing the oil pan gasket, the small line could have been damaged enough that a couple flights later finally created a break in the line. Troubleshoot the easy/cheap stuff first!

[n3833v]

Did you get gasket material stuck in the oil relief valve.

John

[Gmanic]

No, no gasket material anywhere. Should be caught by the filter or screen, right?

We did connect both the Cessna gauge and a second one directly to the case with the same result - no pressure.

No, not 30 psi while cranking, just a few drops of oil out of the nipple fitting.
It is a O-300D in a 1960 172A.
My apologies for posting in this forum, but I know you are the O-300 experts.

Sincerely,
Gmanic

[Bruce Fenstermacher]

It is unlikely, nearly impossible really, for the oil pump to fail so dramatically in just a few minutes. And if it did there would be evidence in the screen.

Also very unlikely, and nearly as impossible for the engine to loose that much pressure in such a short period of time without an oil leak or another catastrophic failure you would surely know about.

I would focus on pressure indication.

Did you remove the nipple that should be in the case where the pressure line attaches and make sure it is open. This nipple is a modified nipple with a very small hole in it about the size of a #60 drill. It could have easily gotten clogged.

The reason for the restriction in this nipple is to restrict the flow of oil if the pressure line should break giving the pilot time to get on the ground before catastrophic engine failure.

BTW we do not mind you posting and asking questions here. We just like to understand how one could remove the oil sump with the engine in the engine mount.

[DWood]

Wasn't there a similar post where the cause was a "Blue Paper Towel"?
That is alway a concern when doing any engine work.

[FredMa]

Yes there was a similar case where a shop towel or rag of some sort was found blocking the inlet screen. Since this problem occured just after maintenance was performed, I would assume the problem is related to that. I also agree that it is not likely the pump as they are so reliable. I would guess a plugged inlet to the pump or blockage in the restrictor as mentioned. I was going to suggest air in the line between the case and the gauge but you have eliminated that by hooking a gauge directly to the case, If I read your post correctly. A pump that has lost it's prime also doesn't seem likely since it was working for a short time after the sump was off originally. I would remove the oil press regulator and the restrictor and clean them both and reinstall. You might be able to simply pull the inlet screen and verify it is clear without having to pull the sump.

[Bruce Fenstermacher]

Here is a list of things I'd do all at once.

Remove both screens and make sure you can not see anything restricting oil flow.
Remove and insure the restrictor nipple is not blocked.
Remove and insure the relief valve is not being held open.
Remove the left front pipe plug in the left oil galley from the front of the engine and pump engine oil into it which will flow back to the pump and prime it.

Reinstall everything and run the engine look for oil pressure.

If still no pressure I'd rig a oil pressure gauge to in the left from oil galley pipe plug and check for pressure there. In effect you would be measuring the pressure of half the system but if the blockage is in the last half you might prove the oil pump is working.

After that I'd be at a lose and probably would remove the sump, not the accessory case as the sump was the last thing disturbed.

[Gmanic]

Thanks for all the suggestions, I forgot to mention that I have the oilfilter plug in adaptor installed also which replaces the fine mesh screen.

[hilltop170]

It sounds to me like something plugging the oil pump intake. Since it ran fine with normal pressure for awhile, it is possible it took some time for the obstruction to work its way into place. If all the other suggestions fail to produce pressure, it would make sense to remove the pan again and check it out. I would not run the engine again other than on the starter until the problem is found. You don't want to wipe out the bearings if they already haven't been.

[jatkins]

This sounds very simlar to my situation,..... with the "blue" paper shop towel.

We changed the sump, due to corrision, around the forward drain plug.
when we removed the oil pan, the gasket broke at the rear case, so we pulled
the engine.

Some how , unknow to myself and the mechanic ( AME in Canada )
a piece of blue shop paper towel rag got into the sump ???????? WTF !!!

I flew the PLANE APPROX 4 HRS, AND THEN ONE DAY DURING FLIGHT
the oil pressure went to 5 psi. I got the plane back to the airport. ( about 3 minutes )
We cecked the guage ,the oil preesure line, and all was ok.

Long story short , just before pulling the engine, we checked the oil piump scavange screen,
and found , 1/3 of a blue paper shop towel wrapped around the screen.

We chnanged the the oil, and now 135 hrs later the engine is still fine.


John

[jatkins]

We hoisted the engine, (if I recall ) about 5 or 6 inches, which permitted us to he sump off.

[GAHorn]

Two things jump out at me.

First: NEVER put compressed air into the oil-filler inlet. You can blow out the prop shaft seal and otherwise overpressure the oil sump/crankcase. (Crankcase pressure is NOT how these engines are lubricated.)

Next: AIR...in the gauge supply line...is NOT a problem. It does not interfere with oil pressure indications. (The air compresses and the resultant-pressure is still indicated at the gauge.)

Bruce has made an excellent suggestion: Remove the lifter-galley plugs at the front of the engine and see if motoring the engine over with the starter produces oil-flow. (Alternatively, completely remove the tubing-flare-adaptor from the gauge supply line at the right/rear of engine and see if oil is produced. Also see if oil is produced at the pressure-plunger when it is removed.)

DO NOT blow compressed air anywhere into the engine system. It will only make the diagnosis more difficult because it will introduce voids into the system and possibly cavitate your oil pump.
Finally, it is not necessary to remove the engine to access the oil pump at the accessory case. You can do that in situ.

[Gmanic]

Thanks for all the suggestions.
My friend and mechanic had already pulled the engine and removed the accessory case from the back.
Interesting to see all the innards.

I should have explained that prior to the oil pressure problem we had the oil pan off to fix an oil leak. A new gasket was placed between oil pan and crankcase.
As the accessory gear case was not removed at that time, a half gasket was manufactured, and attached. We are now thinking that part of that gasket might have been pushed up toward the opening into the oil pan and caused a kind of internal oil leak back into the pan.

The only thing that does not make sense is how oil could have flown form the dead space
Between the oil screens and the oil pump. From there it might have pushed up through the failed gasket into the oil pan. The new gasket showed dark discoloration and was visibly deformed.
(I wish I knew how to post a picture here to explain this a bit better)
Thinking that this was the last thing we changed before the problem occurred, this might be it. Not sure though.
We have now bought a complete gasket set and want to assemble the engine again. The new gasket is a one piece covering the entire back end of crankcase, oil pan against the accessory gear case.
No gasket material, dirt, metal chafing, paper towels or anything was found inside.
Any thoughts before we put things back together?
I would be happy to send pictures to anyone who is interested.

[Gmanic]

Thanks for the answer, we will make sure to follow these directions.