z driver 88t

1984 928S US model

Coming home two nights ago the low pressure oil warning light came on just as I was pulling into my riveway. Oil pressure on gauge reading close to 0. Engine sounded completely normal but I turned it off immmediately.

Let car site half an hour, restarted, gauge read 3 bar.

The next day, cold start, pressure reads normal and gradually on the 10 minute drive to work the pressure starts decreasing as the car warms up. At idel after 10 mintes it was down around 1 bar and would still rise to about 3 if I revved the eingine, but it was definitely decreasing the longer the car was running.

I spoke with two local 928 mechanics who said if the pressure was really close to 0 the first thing I would hear is clacking from the hydraulic lifters. If the engine sounded normal but the gauge was shoing 0 pressure they thought the problem was likely to be the sending unit or something else interefering.

In searching the forums I saw that pin 4 on the 14 pin connector is the signal from the oil pressure sending unit. It looks like it should read about 30 ohms at 3 bar and around 165 ohms for 5 bar. My first plan will tbe to try to get a reading on that pin to see if the sending unit is really sending that signal.

Beyond that - are there any thoughts or ideas? Thanks.

z driver 88t

Update:

Also, I read CraigL's post about what seems to be a similar issue on his car with suggestions on looking at the oil thermostat / thermostat spring.

I do not believe my car has an oil thermostat. My radiator is the one without the oil-cooler, so I am assuming no oil cooler would mean no oil thermostat.

z driver 88t

Solved - It was the oil pressure sending unit that was faulty. On a hunch, knowing that 928 oil pumps are pretty stout and with no evidecne of leaks or fluid loss, in all liklihood my oil pressure was actually OK in spite of the gauge, I ordered replacement oil pressure sending unit.

Problem solved. Now on cold start pressure is high - close to 5 bar then at operating temps at idle pressure hovers right at 3.

It seems that one of the failure modes of the oil pressure sending unit is that as the unit heats up the signal it sends becomes weaker and weaker. Anyway - it was nice to have cheap easy solution to such a worrying problem.

GlenL

Way to go!

THe old electrics in these cars can be so frustrating.

Hkp7

Funny that my 84 just started doing the same thing.

GregBBRD

Hopefully that will trun out to be true. I was just going to explain that the "idiot light" and the actual pressure guage are on two completely different circuits. Yes, they come from the same sender....but they are completely separate...so,
usually when both the guage and the idiot light read alarmingly low....the problem is real.

Let us know what happen when you next get it hot.

z driver 88t

Greg, that is odd. I would imagine that even if the two are on different circuits, they both must get their information from the oil pressure sending unit, unless there is an alternate source of oil pressure signal being sent to the light, which I would doubt.

In my situation the guage was responding as if the oil pressure was low. It would idle at about 0 once the engine was warm and the light would be on. If I revved it or kept the revs above 1500, oil pressure on the gauge would climb to about 2 and the light would go off.

Pressure sending unit replaced and all symptoms are gone. I've took the car on a 45 minute drive on Wednesday in FL summer heat - so its good and warm. Gauge and light respond the way they should and have been driving it daily since then.

BTW - the failure was very sudden. Guage / oil pressure showed fine for the past 4 years. Then one night it just dropped to 0 coming home. I've never busted open an oil pressure sending unit, so I don't know if there is some sort of internal strain gauge or diaphragm or what inside would specifically fail.

However, the net result in my case was that all previous symptoms are gone. YMMV.

z driver 88t

Hpk7 - the job is not too bad. Do a google search and there is a writeup on replacing it with pictures. You need to remove the oil filter to have room to get a wrench on the sending unit, so just plan to do an oil change at the same time.

MFranke

Yeah, the oil pump is pretty stout I think. I recently opened the pump up on my 88 S4 at 170k miles. It looked almost new with hardly any wear on the gears.

Weissach77

Thanks for the post! My 82 is having same problems! When starting nice pressure, however as I'm driving it drops.

ROG100

Weissach77 - welcome to Rennlist.
Oil pressure will drop once the oil reaches temperature. As low as maybe 3 bar.
Are you seeing 0 pressure - you warning light will come on at less than 1 bar pressure.

I keep a record of Weissach cars - let me have your details at Roger@928srus.com please?