Hkp7

Well, I think I finally found something on my 84s running rough. I put an oil pressure gauge where sending unit goes and I am getting a reading of 120 psi. I have done a compression test on all cylinders and they were all 150 psi. dash gauge needle has been pegged for months and even before it ran rough. I thought the sending unit was bad.
Is a head gasket leaking compression to an oil passage? Is there a typical blockage from pump to rest of engine?
What are your suggestions? I appreciate them.

Hilton

Sounds about right for a 928 engine at cold idle.

The oil pressure bypass opens at 8 bar, so you should see 8 bar max oil pressure.

As it warms up, you'll get lower, but under rpm's it'll push 8 bar again.

Hkp7

Where is the oil pressure bypass. Also in the PET it is showing springs and a plunger above unit that holds sender. I didnt have those parts in mine.

IcemanG17

150psi per cylinder is pretty good......the current engine in my 84 racer averaged about 145psi...and dyno'd strong at 203whp....

I would also say your oil pressure is normal...especially cold......my gauge only reads to 100psi and its PEGGED until oil reaches at least 140F..... Even at very hot oil temps approaching 270F on track I can still reach near 100psi at 6000rpm....

In general 928's like lots of oil pressure......compared to other engines anyway...for example the Opel GT I race would reach about 35psi MAX oil pressure which is about the same for Miatas too

Hkp7

The manual is saying 5 bar at 5000 rpm, that would only be 73.

Speedtoys

Thats a good minimum...sure. But..its not why yer runnin rough.

IcemanG17

That is the minimum spec..... I NEVER see less than 75psi above 5000rpm.... it typically pegs 100psi above 5000....

High oil pressure shouldn't cause poor running...it would have to be seriously high..chances are your rough running is due to an old component failing over time....so many things to check:

1: Spark plugs
2: plug wires
3: cap and rotor (even Dist)
4: infamous green wire to ignition box
5: temp sensor
6: AFM
7: fuel filter-pump

Can you describe in better detail exactly when and how it runs rough?

Hilton

The problem is the manual assumes you only have the dashboard gauge to go by, which only indicates up to 5bar.

The oil pressure relief valve (sometimes known as pump relief valve) prevents the journals seeing higher than 8 bar, but unless you fit a separate gauge like you've done, you have no way of knowing if the indicated 5 bar is actually 5.1bar or 7 bar or higher.

Dennis Wilson

The 928 needs higher pressure readings due to the location of the sensor. Most cars have the sensor located at the far end of the oiling passage (rear end of head) but the 928 has the sensor next to the oil pump, before there is any pressure loss from the rods, mains, lifters etc.

Dennis

Hkp7

My regulator and springs are missing. On order. Thanks for everyones help. Bill

Eric Buckley

The regulator and springs are part of the oil thermostat associated with the oil cooler, which you don't have. In fact, installing these parts on a car with no oil cooler is a very bad idea, as it will result in an reduction in oil pressure since the oil has to flow through an extra bypass designed to handle a blocked oil cooler. See this thread for some discussion.
https://rennlist.com/forums/928-foru...questions.html

James Bailey

Correct 1980-1984 USA 928s have no heat exchanger in the radiator so no need to reduce the pressure(bypass) to avoid blowing up the oil cooler and not knowing it since it leaks into the coolant. Later models 1990 > with air to oil cooler do not have the bypass since it is obvious when the cooler leaks Ugly but obvious !

Hkp7

I think I have made a mistake on the terminology. I was referring to the valve and springs in the round cup that the oil sending unit is mounted to. These parts were in there when I replaced the sending unit and have mysteriously been missing since I had the car worked on at a local shop to do work I didnt feel like doing do to cold weather. I also found that my bowden cable was out of adjustment after going there and they wanted an hour labor to adjust it. It was adjusted before I went there. Same people told me my cooling system held 8 gallons and that is why they were charging me 15 dollars each for four gallons of coolant. This is one of the reasons I would prefer to work on my own car. I would have a hard time trusting any shop at this point. The running rough problem has been frustrating. I have had two people who specialize in 928s as well as two other very experianced auto mechanics look at the car since. I always get the same answers that they need to think about this one.

Eric Buckley

Does your car have an oil cooler or not? If it has an oil cooler there will be two oil lines attached to the block, directly above the oil pressure sender. If there is no oil cooler then there are threaded plugs above the sender, in the holes where the oil line fittings would go.

We are all talking about the same parts. With reference to the drawing in Post #1 of this thread:
https://rennlist.com/forums/928-foru...questions.html , the 3 parts highlighted with a white background are the two springs and piston you are talking about. If you have no oil cooler you are not supposed to have these 3 parts highlighted with a white background installed (this is the oil thermostat). You are also not supposed to have the rearmost spring and piston shown circled in red (this is the blocked oil cooler bypass). The only spring and piston you should have in this area are the ones circled in red towards the front of the engine (this is the maximum oil pressure limiter). Referring to the picture in Post #8 above, if you do not have an oil cooler you should not have items 4 and 5 installed.

Regarding the 120psi reading; that sounds exactly right for cold oil or higher rpm's. The maximum oil pressure limiter should be opening at about 8 bar, and 8 bar is about 118psi.

It can be very difficult and frustrating to find a competent 928 mechanic, and you know it won't be cheap. This is why I do all of the work on my own car.

Your rough running could be due to many things, as you know. You should start (another?) new thread on the rough running. I have a similar car to yours (mine is a U.S. '83S) and I find that I need to replace the distributor and associated rotor annually to keep it running well. An aging AFM is also a possibility, 928Intl does sell rebuilt units.

Keep working away, and asking questions. You'll get it running the way it should!!

Hkp7

Ok, the car has no oil cooler. I thought the regulator was in one of the closed off ports above the oil filter.