low oil pressure when warm

[w113dude]

Hello everyone,

Have a freshly rebuild 250sl engine, the engine runs perfect with the exception of the oil pressure showing low when warm, the oil pump was verified, all the bearing tolerances were within range and the engine oil is 15 40, my question is do the oil pressure valve or the oil gauge ever go bad?  or I should ask what is the likelihood of them being faulty?

I thought there might be some small debris stuck in the pressure switch valve. has this happen to anyone?

Thanks
Shaun

[Benz Dr.]

Oil pressure should be full above 1,500 RPM. Most engines will show full on the gauge when cold and drop a bit at idle when fully warmed. Tell us what you mean by low oil pressure.

[w113dude]

when cold shows full 3 warmed up the oil pressure drops to 1.5 at idle when in gear even lower,  goes up to full to 3 at 2800 rpm, the oil is nice & clear no sign of wear in the crank and no knocking noise, although it's been very hot these days I have never had anything like this in any of my M129 or M130 engines

[ja17]

If your engine oil pressure continues to de-grade you could have a problem. It is lower than normal. I think I would drop the oil sub pan and do an inspection of the oil pump screen and pan to see if you have any metal particles.

[teahead]

What SHOULD it be at idle?  Both in N/P and in D/R?

[w113dude]

Joe,

I initially thought that may be the problem as well so I changed the pump with a clean screen. no change, I spoke to a few mechanics and they tell me its normal in very hot days.
I should also add if the engine cools off for about half hour then the pressure stays up at 2.5 till it goes for a drive

[ja17]

1.5 or a bit lower at idle with warmed engine is not unusual. Dropping off the peg at 3000 rpms is unusual. It may just be a result of more bearing clearances from the machine work, worn oil pump, or some bearing damage occurring during break-in period. I have seen debris get stuck in the oil pressure relief valve also. If your oil pressure begins to degrade, you will have to look into it.

[w113dude]

Thanks,
I will look in to the pressure valve, the bearings were checked with plastigauge all within range, i had the crank cleaned  all passages were cleaned and correct torque.
Just to make sure I'm covering everything I will first replace the oil gauge since I have the cluster out.
will keep you posted.

[ja17]

The oil pressure gauge is usually "bullet proof".

[Benz Dr.]

Oil pumps create volume and oil pressure is created by how tight all of the tolerances are within the engine. The more wear you have on your bearings the lower the oil pressure reading. Oil pressure should stay at full when cold and close to full or above even when fully warmed on a fresh rebuild.

[w113dude]

Update on the low oil pressure, since I had a spare oil gauge I replaced it there was no change, I also replaced the oil pressure valve in the front of the engine that didn't do anything either, so after speaking with the guy who polished the crank we decided to leave it the way it is as it drives very strong, most likely one or two of the bearing have a bit more gaps than usual.  I will be driving the car regularly if anything changes I will notify.

Thanks for all the help.

[UJJ]

Hi Shaun,
what is the oil pressure valve in front of the engine?
Thanks, Urban

[teahead]

was the engine broken in with DINO oil?

Hopefully.

I'd run synthetic.  Maybe slightly thicker than normal if not driving in the winter.

Sending unit ok?

[Benz Dr.]

Hi Shaun,
what is the oil pressure valve in front of the engine?
Thanks, Urban

That is a spring loaded valve at the front of your engine designed to bleed off excess oil pressure particularly when oil is cold during or right after starting your engine. If you didn't have this valve, high oil pressure could literally blow out gaskets and seals or damage internal engine parts like your oil pump and drive gears.