Calling all oil-pressure experts!

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paul.adshead

Member
Posts
23
P38 4.6L V8, is having oil pressure weirdness at idle. Details in the video below, showing the hot-idle oil-pressure cycling between approx 29psi and 12psi. Any ideas of causes or things to check?

 
brianp38dse said:
I remember the thread well, it was a bit of a bugger and had you head scratching a lot and you did give it a good go but like some things a fresh set of eyes and all was clear.:(
Click to expand...
It did cause some pause for thought thats for sure....it was overlooked because I had changed it out...admittedly (and this is what the issues was in the end) I changed it for a 'similar sized' O Ring - and not a genuine one....I assumed it would have worked so discounted the O ring as the culprit.....but that is indeed what it was in the end - so well done to @Grrrrrr and @MrGorsky for figuring that one out!
 
Saint.V8 said:
It did cause some pause for thought thats for sure....it was overlooked because I had changed it out...admittedly (and this is what the issues was in the end) I changed it for a 'similar sized' O Ring - and not a genuine one....I assumed it would have worked so discounted the O ring as the culprit.....but that is indeed what it was in the end - so well done to @Grrrrrr and @MrGorsky for figuring that one out!
Click to expand...
What were the symptoms? Did the pressure jump about like this one? Or stay low?
 
@Saint.V8 Did you happen to note the correct o-ring size? Then I can get one on hand before opening it up. I was in the sump a couple of years ago when I changed the big-end bearings, but the o-ring still seemed very snug at the time. I should have changed it I guess.

 
paul.adshead said:
@Saint.V8 Did you happen to note the correct o-ring size? Then I can get one on hand before opening it up. I was in the sump a couple of years ago when I changed the big-end bearings, but the o-ring still seemed very snug at the time. I should have changed it I guess.

Click to expand...

Perhaps you should have changed the main bearing as well,when you have the engine running do you have a rumbling noise ,sure sign that the mains are shot and will drop the oil pressure.
 
There's no rumble. At ~1200rpm it's 40psi, at 1500rpm it's 55+psi, occasionally see it at 60psi but it doesn't go above that. I had considered doing the mains, but I believe doing so has a really high likelyhood of damaging the rear main oil seal (- replacing that is a HUUUGE amount of work).

The fact it sits at 12 or 29psi at idle, is almost like it's switching between two modes. I guess it could be the o-ring leaking, letting air from the sump into the pump, partially 'loosing it's prime' and dropping the pressure. After it has been run at higher RPM, the increased splashing in the crankcase, might be sufficient to coat the o-ring area, temporarily stopping the leak. Then after a minute or so of idle, the splashing is less, the excess oil has drained away from near the o-ring area, and the o-ring is then leaking air into the pump. I don't know, I'm just trying to make sense of the gauges I can see.
 
Last edited:
paul.adshead said:
There's no rumble. At ~1200rpm it's 40psi, at 1500rpm it's 55+psi, occasionally see it at 60psi but it doesn't go above that. I had considered doing the mains, but I believe doing so has a really high likelyhood of damaging the rear main oil seal (- replacing that is a HUUUGE amount of work).

The fact it sits at 12 or 29psi at idle, is almost like it's switching between two modes. I guess it could be the o-ring leaking, letting air from the sump into the pump, partially 'loosing it's prime' and dropping the pressure. After it has been run at higher RPM, the increased splashing in the crankcase, might be sufficient to coat the o-ring area, temporarily stopping the leak. Then after a minute or so of idle, the splashing is less, the excess oil has drained away from near the o-ring area, and the o-ring is then leaking air into the pump. I don't know, I'm just trying to make sense of the gauges I can see.
Click to expand...
Bite the bullet and get an oring from the stealers you'll then know it's the correct size and more importantly the correct material.probably very expensive as I found out when I got the oring for the heater take off pipe by the water pump on the diesel cost me£4.80 from memory :eek: do it once and do it right;):D
 
kermit_rr said:
Pretty sure there's a sump gasket :eek:
There is on the earlier version and later version.. the blocks are all the same
Click to expand...

It depends on the sump material I believe. RAVE says to use sealant.
sump refit.jpg
 
What work has been done to the car recently?

It isn't a massive job taking the front cover off. I might be tempted to take it off, clean and flush everything carefully and then oil and filter change. Dropping the sump and changing that o-ring on the pick-up won't take that long. Just make sure the car is on HIGH so you have room to manoeuvre the sump out of the way of the drag link etc. If it still does it I would be thinking oil pump worn.
 
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