1982 Porsche 911 Engine oil Pump or bad sending unit?

[C.huish123]

Car Details: 1982 Porsche 911 SC 3.0 Liter, 182k original miles but Motor rebuilt 25k miles ago but has original oil pump that was cleaned during rebuild (not replaced).



Question:Over the past few months I noticed and researched that the oil pressure druck press should be about 1 Bar every 1k RPM. During cold start it will be about 2 Bar and than when fully warmed up it will be at 1/2 bar which seems low to me. Even if I go to 5k RPM the max it will go up to is 3 Bar. So I was thinking maybe something could be wrong with the oil pump. I disconnected the top wire to the Oil Pressure sending unit and when I turned the ignition just so the radio/electric was on (not starting the car) the Druck press went to 5 Bar. I took some fine sand paper to clean the connection on the sending unit and the wire that is connects and then reconnected. I drove the car about 1/4 mile yesterday and all of a sudden my oil pressure went to "0" so I turned the car immediately off - It has never done this until yesterday after I sand paper the connection but regardless before that the druck press gets max 3 bar even if I am at 5k RPM .

My 1st Question: Based on your knowledge and opinion can you give me an educated guess if maybe I have a bad oil pump or bad sending unit? Also, I noticed the Oil Pressure "dummy light" never came on when it entered "0" oil pressure, which makes me believe the oil pump might be okay because the light works when I half turn the ignition.
My 2nd question: if you disconnect the top wire on the sending unit and its not touching any other metal, shouldnt the druck press read "0"?


thanks

[Walter_Middie]

C.huish123,

If your engine wasn't making horrible noises, then your oil pump is fine. Typically low oil pressure is NOT caused by the oil pump. It is caused by wear in the engine bearings.

It is my opinion that the oil pressure guage is probably the worst gauge to put on the dash board. It doesn't tell you anything you need to know, and it can only cause worry - such as you are having now.

Since you were playing with the sensor wires when the gauge quit working, I'd say the problem is most likely the wire connection. What happens when you ground the wire? Does the gauge go to full or zero deflection?

[C.huish123]

Rex - I just checked and I disconnected that top wire again that connects to the Oil Pressure sending unit and made sure it wasn't touching any metal and it went up to 5 Bar again...I did what you mentioned about, what happens when I ground the wire? When I touched it with another piece of metal it stayed at 5 Bar, BUT when I screwed back on that top wire I turned the ignition a half turn and it went slightly above the 0 mark on the druck press.

Hope that gives more insight to the diagnosis. Also, there was no grinding or anything like that when I was at "0" while the car was on, but I spoke to a BMW/Porsche Auto parts store and he said that you wouldn't neccesarly hear any grinding if there wasn't any oil pressure.

[Walter_Middie]

when I screwed back on that top wire I turned the ignition a half turn and it went slightly above the 0 mark on the druck press.

Does it appear to work normally with the car running again? It might have just come loose.

you wouldn't necessarily hear any grinding if there wasn't any oil pressure

Nope - no grinding. Just a lot of clattering. You'll know it if you have no oil pressure, the engine will make a lot of noise. I suspect your oil pressure is fine, you just have a bad connection. My recommendation is to get the gauge working again, then ignore it.

[Gogar]

1. When you disonnect the wire, it is proper that the gauge to to 5.


The best way to stop worrying is to buy a manual hydraulic pressure gauge and fittings to fit where your pressure sender goes, and you can start it up and look at the pressure from the back of the car.

There's lots of failure points for the sender, the wiring from the sender to the gauge, ground points, and the gauge could be sticky, too.

It's highly unlikely that your oil pump is bad.

[kidrock]

a compression test or leak-down would probably give you a wealth of info.

[dshepp806]

I agree,..simply measure the oil pressure at various RPM for a piece of mind on the engine side.

Doyle

[Walter_Middie]

Your next question should be – what is the oil pressure gauge telling me? And why should I ignore it?

The following is true for any car – not just Porsche’s. When you first start your car cold, the oil is thicker and will have more pressure. As the oil warms up, it gets thinner and the oil pressure drops. The oil is formulated so that is works best at a normal engine temp – not too hot, and not too cold. That’s why you drive your car easy when it’s cold – because the oil is not up to temperature, and is too thick and not flowing as easily as it should. At the other extreme, if your engine overheats, the oil can break down and not protect the engine as it should. That’s why you don’t drive a car that is overheating, even though it may still be running – temporarily.

Once the engine is up to normal temperature, the oil will coat and protect all the metal to metal surfaces – such as the crank bearings and the crank shaft itself. The oil pressure is a function of the clearance between the surfaces like the crank and its bearings – the oil pressure is not a measure of the oil pump wear. As the engine wears with normal use, the crank bearings will wear, and the clearance between the bearings and the crank will get larger. This will cause the warm oil pressure to be less and less over time.

Ideally, the warm oil pressure will be about 10 lbs per sq inch (PSI) for every 1000 RPM, so that at 3000 RPM, the oil pressure will be 30 PSI or about 2 bar. This is only a rough estimate and can vary. If your pressure is less, it simply means your engine is exhibiting normal wear.

If your engine pressure is zero, this is a serious issue. However, over time and miles, your engine can wear to the point where the oil pressure is essentially reading zero at idle, and only a few pounds at higher RPM. This is still OK. Your engine won’t have catastrophic failure unless you push your car to extremes, like driving it in a race, where you punish it lap after lap. For everyday street driving, the oil pressure can be very low and still provide enough oil protection to coat all the surfaces and avoid serious engine damage.

The reason why I say the pressure gauge is not something you should look at everyday is that your street car engine will wear over hundreds of thousands of miles. The oil pressure gauge only needs to be noticed every 50 to 100 thousand miles to get a feeling for the amount of bearing wear. Looking at the pressure gauge all the time, and going from cold to hot does not really tell you anything, and is why I think the oil pressure gauge should not be on the dash board. Maybe it should be mounted in the engine compartment so you can observe it when you do a tune up every 10 or 20 thousand miles. Mounting it on the dash board only leads to needless worry as it move up and down with engine temp. That’s perfectly normal.