Fake oil pressure, a cautionary tale

[Dearg1275]

Don’t be caught out by this one. As usual I fitted a new oil pump, primed with thick oil, to my engine rebuild. With the engine back in the car and ready to go I cranked it over without plugs to get the oil pressure. It took a while but I got 50 psi. Great.

Next day I went back to see about starting the car which it refused to do. Just as well, as when I looked there was no oil pressure. With the plugs out again I tried - no result. I back fed the pump and tried again - zilch.

So what happened? The oil pump gasket had not compressed by the inlet port and was drawing air. I had filled the oil filter with oil. The pressure I was getting was AIR PRESSURE while the pump pumped air displacing the oil in the filter canister. Once that was gone there was no longer any resistance, so no apparent oil pressure.

To fill or not to fill the oil filter. That is now the question.

D

[GraemeC]

The question is why had the oil pump gasket not compressed and sealed - did you verify the bolt lengths?

Once that’s sorted, I always fill the filter and prime the pump before fitting and if it’s stood a while after building then prime again before proving oil pressure prior to starting.
I guess if you’re concerned there may be an issue you need to leave it a day and then prove pressure again, effectively as you’ve done here.

[woodypup no 2]

Had something similar when the oil pump didn't seat properly. Turned out to be adverse tolerences between cam and oil pump. Cured by angle grinding the notch in the pump deeper.

[Polarsilver]

I smear the Oil Pump Gasket in Grease to hold it in position while thinking this will help the gasket seal.. i always pre-fill any new oil filter before fitting.
Can understand your air pressure reading as i always wonder about the air trapped in the oil pipe supply to the Pressure Gauge.. that oil presssure capillary pipe does not fully clear its air ever best as i can see

[Peter Laidler]

You are sort of right to be concerned about the air that might be present in the oil pressure line to the gauge. You'll never (?) clear it of air because try as you might, it is at the highest point in the car (so far as the oil is concerned) and air will ALWAYS migrate upwards. But I wouldn't worry about it none - as Bob Dylan said (can anyone tell me the song the phrase came from.....) because when the engine is runnin' and the pump is a pumpin' the air in the pipe is compressed to that figues. Same occurs when there's a bit of air left in the hydro system

I suppose you could bleed the oil gauge pipe by loosening it half a thread at the gauge end, turn the engine over until oil oozes out of the loosened end, then lock-up

The same featured on tank engines of the low pressure but high cappacity type except that there was a bleed tap behind the gauge cluster.......... JUST out of the sight of the tank crew so only the teccies could get to it.

[rolesyboy]

Bob Dylan 3rd World War Blues Pete.
Great song

You are sort of right to be concerned about the air that might be present in the oil pressure line to the gauge. You'll never (?) clear it of air because try as you might, it is at the highest point in the car (so far as the oil is concerned) and air will ALWAYS migrate upwards. But I wouldn't worry about it none - as Bob Dylan said (can anyone tell me the song the phrase came from.....) because when the engine is runnin' and the pump is a pumpin' the air in the pipe is compressed to that figues. Same occurs when there's a bit of air left in the hydro system

I suppose you could bleed the oil gauge pipe by loosening it half a thread at the gauge end, turn the engine over until oil oozes out of the loosened end, then lock-up

The same featured on tank engines of the low pressure but high cappacity type except that there was a bleed tap behind the gauge cluster.......... JUST out of the sight of the tank crew so only the teccies could get to it.

[Peter Laidler]

Is that the song that continued '.....them old dreams are only in your head....' 10/10 Roly. Great song and musician.Talkin' blues just shows what a great musician he was too.

Have a great Christmas break too. And the same goes for each and every Forumer worldwide, from Pete the Pom

[Spider]

Nice and lucky catch !

I always check the Oil Pump Bolt Lengths by first removing the washers from the bolts and making sure they go all the way to hold the pump firm, then I know when the washers are fitted, I have at least the washer thickness up my sleeve here.

In regards to filling the Oil Filter, I've never done that, only the Pump. If the engine is to be fired up shortly after the build, I'll just use engine oil in the pump, if it's to be stood for say a month or longer, I'll use gearbox oil, though I don't like doing that for a variety of reasons. One I did, due to Covid, stood for about a year before I could get to it. I removed the Oil Switch, made a fitting to go in there and pre-primed the bottom end by pumping oil in to the main gallery from here and before cranking it to prime it in the usual way. This did back-fill the filter of course and so needed a fair bit of oil before seeing some pressure.

[Dearg1275]

No it wasn’t the bolt lengths. I always check that. It seems that the fit surface of the pump was distorted and will not be going back on the engine. Here is a picture of the gasket immediately after the pump was removed. It is a two bolt pump with the casting reliefs. I will be fitting a four bolt with a solid fit surface to all my engine rebuilds in future.

BAFC84D3-9373-4C27-938B-B7033CB1D0A7.jpeg

I do wonder if it is worth making a trial fit with a gasket to check compression before final assembly with a fresh gasket.

My concern was not about air in the gauge line. It was that the pump could generate air pressure in the system enough to show 50 psi WITHOUT PUMPING ANY OIL !

D

[mk1]

BUGGER!

I fell foul of the bolt length thing earlier this year, having (for once) all the original oil pump bolts I thought I was safe. . . . I wasn't. Still, cock up rectified & all seems OK now.

I have never seen this problem before, but have never been a fan of 2 bolt pumps.

[Spider]

Yeah, looks quite light on for pressure on the gasket.

Does anyone do a standard flow 4 bolt pump these days ?

[floormanager]

An interesting video here

https://www.youtube.com/watch?v=-ogmAuY7VSA&t=474s

[Spider]

An interesting video here

https://www.youtube.com/watch?v=-ogmAuY7VSA&t=474s

For a pleasant change, I felt that one is worth taking notice of, the guy knew his stuff, explained things well and wasn't a bull in a china shop.

[Costafortune]

as Bob Dylan said (can anyone tell me the song the phrase came from.....)

Pissin' in the wind?