Thread: V8 oil pressure mysteries

Hi all,

I need ideas from V8 gurus around here about oil pressure in the V8.

History :
3.9 V8 serpentine, 192000 km, rebuilt 5 years ago when car bought, stock engine in auto disco 1.
Fitted an oil pressure gauge since the rebuilt to keep an eye on it.
Running on castrol edge 25w50 as I am living in a warm climate and using genuine LR oil filter.

It has been looking perfect since then : 40 psi whatever the rev when cold, then 20 psi minimum on warm idle, and would climb up with the rev to 40 psi from about 2000 rpm and will stay like this on higher rpm.

So all looking good up to now.

Then, after changing the viscous fan which was engaged (and noisy) all the time... it appeared that the engine then wanted to run pretty hot, yet (as I have also a coolant temp gauge) I Never let it overheat.
After changing several parts, step by step, it appeared that the stock old rad was surely clogged inside, as installing a new rad cleared the hot running issue.

BUT !!! since the new rad (it is a NISSENS, plastic/steel one)... the oil pressure does not reach 40 psi anymore !?!?

Just before switching off the engine to swap the rad, the oil pressure was acting as stated above.
Changed the rad, refilled, restarted, and from then on :

30 psi when cold, then 20 psi minimum on warm idle, and would climb up with the revs up to 30 psi... but will never go above now !

So I am wondering a few things :

-> Is there any bleeding necessary of the oil cooler circuit when replacing the rad ?
(not much oil leaked from the oil lines I had plugged on disassembly, but the new cooler was empty from oil after rad replacement)

-> I believe the oil pump is pushing the oil into the cooler and then into the filter and finally to the engine... but where is the pressure taken exactly (my gauge takes the pressure at the same location as the stock pressure warning switch) ?
Is this just after leaving the pump or is it after cooler and filter ?

If pressure taken after leaving the pump, could it be that the new cooler is more "free flowing" than the original (clogged) rad, meaning that with less back pressure from the rad, the pump builds less pressure but with a greater flow ?

If pressure taken after the cooler and filter... then I don't know...

Well, could anyone please help me understand and give me clues on :

=> why has the max oil pressure lost 10 psi just after swaping the rad and nothing else disturbed ????

=> do I have to worry about this behaviour and what should I do about it ?

Thanks for feedback in advance

Best regards

Erik.

Hi Bee utey,

thanks for your answer... I was afraid someone would say this...

I am pretty sure I have not forgotten any plug, as I remember having a look inside the oil cooler connecting ports... Damn'it...

So that may be the Nissens "drop in replacement" rad is more flow restrictive than the original copper one, producing an oil pressure drop accross it !
And consecutively, an increase in pressure at the pump before the cooler, making the pressure relief valve operate without oil pressure at the engine being the same as before.

Which means NISSENS RAD = additionnal 10 psi drop accross its oil cooler... bad news...

As It will be hard for me to change rad again, would it be sensible to kind of add a little washer at the back of the oil pump relief valve, in order to stiffen the valve spring a little and increase the pump delivered pressure...

Would this be ok or would this put detrimental stress on the oil pump for un-necessary gain in oil pressure at the engine ?

Thanks again

Cheers

Erik.