I have "searched" the forum, and found a few helpful ideas, but felt I need to post my dilemma.

I have recently acquired a second 964, this time a 1989 C4 coupe. Car has 150k miles on it but the engine was completely rebuild 3k ago. IT included a new oil pump, new temperature sender and new pressure sender. Pressure sender is listed on the itemized list at $37.20 so not likely OEM.

I've had the car a week with a trip permit. I drove it once for about 20 miles with no drama. The next time I drove it, I came to a light after 15 miles and the car went to idle. oil pressure symbol and "!" came on the dash. Revved the engine for 5 seconds and the alarms cleared. But the pressure indication dipped to the zero range, not just to the 0.5 range. This did not occur when I test drove the car, nor did it occur on the first 20 mile drive I did.
I've reviewed the maintenance sheet which shows it was filled with 20w-50 Castrol GTX oil, although the window sticker shows 20w-40C (?).

Engine rebuild was done at a Porsche Tuning shop in Calgary Canada.

At first start, the car builds 2.5 bar pressure cold, but I'd guess that this has dropped off after 3 miles of driving. Cold, I can get 5 bar above 3k rpm, hot, I'm getting about 3 bar at 3k rpm.

One reason I pulled the trigger on this car was due to the overhaul on the engine (I have $28k in receipts) so I really really don't wish for engine work.

My thoughts:
1) change oil and filter. Change to what?
2) change pressure sender an o-ring
3) get a good divorce lawyer???

Any and all help, comments, jabs are helpful to me at this point.

BTW, my '91 C2 cabrio shows none of these issues. Hot, it runs at 2 bar all day long. I have 15w-50 Mobil synth in it.

Peter

 

2.5 cold is low too.

 

Definitely too low.....before you drive it any more swap the oil pressure sensor and see if you have a faulty sensor.

 

I would get the car to a shop you can trust to verify what the oil pressure really is. I would remove the oil pressure sensor and swap it for a known good one. I would also confirm what oil you have in the engine. If its too thin, no knowing what damage that can cause.

 

There is a shop here I can trust and I'll see about the pressure. I was hoping for a naaah, no worries, but my gut seems to tell me what you guys are suggesting. I have called the tuning shop in Calgary, and the head mechanic is to call me back later today to discuss.

 

Does anyone have experience with the oil pressure relief valves getting stuck in the open position and causing the oil pressure to remain lower due to dot closing? Looks like the 964 has two of them.

 

Quit stressing and wondering. Replace the oil pressure sending unit.
My coupe had the same symptoms out of the blue a month ago. New sending unit, and all is good now.

 

Type911,
Did you go with the VDO OEM brand?

I've searched enough on this topic to find that there are many people that experience the same symptoms, but have found varying causes.
Pressure relief valve stuck
Pressure sender unit faulty
Oil pump plugged
Oil pump sealing rings pinched during reassembly
Etc

As you can likely tell, I really don't want to tear down the engine if it isn't necessary. So I'm thinking the surface issues first

 

Yes, VDO oem from Sunset Porsche.

The oil pressure gauge for the last 3 years would read properly but the needle would bounce 2-3 bars at idle. Finally it quit and started to read very low. Upon removal I found the sending unit was aftermarket and not VDO.
My money is on the sending unit...

 

I have ordered a new VDO sender unit. I'll try that first, along with checking the pressure relief valves and pressure safety valves to see if they could be stuck open and dumping to the sump.

 

So I'm attempting to rule out massive engine work during this process. So I ran a test:

Started the engine and allowed the idle to settle. Somewhere around 850. Started my watch. Oil pressure was 2.5 bar upon startup.
So I watched with antici-----pation (rocky horror reference). Oil pressure held at 2.5 for about 3 minutes and then slooowly dropped. It took almost 15 minutes for the oil pressure to drop until the oil alarm light came on at approx .5. So, I'm thinking that the oil pressure drop is a function of the oil temperature with all other things being equal. If this were to be a physical problem, would I not have the issue right off the bat? Say for example there was a problem with the oil pump, would I not have the low oil pressure immediately. And why the slow but steady pressure drop?
So my plan going forward is change the oil and be sure that it is 20w-50. During, I'll pull the safety and pressure relief valves and check they are not stuck. Failing that, I'll swap the oil pressure sender with the OEM VDO BRAND. After that?

 

Sounds like a good approach. Keep us posted. Hopefully, you won't need a "what's next" option.

 

Oil pressure falls as the motor comes up to temp, except it should start at around 4 bar at idle cold. That's been my experience.

 

When you pull the pressure sender, put a mechanical gauge to monitor the pressure at warm idle.

Depending on where you are, I was able to borrow a gauge from Autozone.

 

Sounds like a good idea. Does the gauge screw into the pressures sender plate? Are there are few adaptor pieces I'll need to source to set it up? If so, any idea what the thread is in the sender plate?
I'm planning to go with CASTROL GTX 20w-50 oil for this test. Any concerns?