eniac

A customer of mine called me over the weekend telling me that his car isn't showing any oil pressure for about a minute after he starts it. The low oil pressure light is on and the needle doesn't move. He said after a little over a minute the gauge suddenly pops up and reads fine. I told him not to drive it anymore like that. I was able to take a quick look at it to make sure it wasn't an electrical issue, it's not. The car really doesn't have any oil pressure when cranking.

My thought is that it's the oil pump losing it's prime and needs to be rebuilt/resealed.

Could anything else cause this to happen?

Could an OPRV stick the other way giving no pressure?
Cracked oil pick up tube? I doubt it's this since he says the oil pressure was fine after it warms up.

It's a 1986 951 btw.

Spidey944

oprv - 99% sure. Mine was doing it too. After the car sat for an hour or so, the pressure would bleed off. I removed the 3 piece OPRV, replaced with another 3 piece unit I had laying around, and the problem went away completely.

eniac

Hmm ok thanks. That's what it's doing. The OPRV is much easier to replace so I will do that first and see if it builds oil pressure, if not then the oil pump seals.

joonas

Is oil level at max mark?
Does he use original oil filter?
Check for proper tension on the crankshaft pulley bolt.

M758

replace the oil filter first. Filters that break their anti-drain back element can cause this. If that does not work then go after the OPRV.

joonas

I think that pressure bleeds off in seconds.

Spidey944

I guess what I was trying to explain was that after sitting any MORE than an hour it would take 15 seconds or so to build up pressure after it was run to opperating temp. If I only took a short drive, it would hold pressure for a few hours because the OPRV never opened up. It would always bleed down overnight and have extened oil pressure build up time.

Zero10

any pressure is gone right away, doesn't even take an hour. the OPRV never 'opened up'?... it is pretty much always open a little when the engine is cold, or whenever the engine is above 3000-4000RPM.

Delayed oil pressure on startup can be a few things
1) oil filter, if the drainback valve is broken or not there this can be the issue
2) OPRV, they can stick open, but it is pretty rare to fail like this. That said, it's pretty simple to check so its #2
3) broken pickup tube, more common than you would think, complete PITA to get to. Hope this isn't your issue

4) oil pump, I can count on one hand how many of these I have seen fail, and that is "fail" in an internet forum sense, not actual conclusive proof the pumps were bad. I only know of one that was bad for sure. Pretty rare failure.

Somebody correct me if I have missed anything

eniac

Oil is full, New Mann filter. I haven't check the crank pulley but I will add that to the list of items when I go back to fix it(The car is 2 hours away from me).

It's a new filter but even with a bad drain back it should take 10 seconds at most to read some pressure.

Do you mean yours actually held pressure when it was shut off or the pressure still went to 0 but the longer it cooled off, the longer it took to build pressure back up?

Wouldn't a cracked pick up tube give zero or varrying oil pressure readings? The oil pressure is steady and normal after it 60-90 seconds.

I know the pumps themselves rarely fail but it's possible the sales can develop a leak which would cause the oil pump to lose it's prime so I was thnking it may need to be resealed.

Spidey944

The longer it cooled off, the longer it took to build pressure back up. Just replace with a known good valve and be done with it. None of your symptoms point to a pick-up tube or a pump issue.

joonas

Right oilfilter is mahle OC-142 (944 83-91 incl S, S2, Turbo).

And like others allready suggested OPRV http://clarks-garage.com/phpBB2/view...oil+pump+prime

Mrmerlin

could the crank bolt be loose this clamps the oil pump drive