I am trying to diagnose a low oil pressure problem on a LS motor. It's a forged 383 built buy a reputable builder. On cold start the oil pressure is 67 psi, as the oil warms up to 200 degrees the pressure drops to 30 psi. At 1800 it climbs up to 54 psi, 3000 it's up to 60 psi then when go WOT oil pressure rises up to 4700 rpm then drops to to 40 psi and holds that around 6500.

The oil pump is a Melling 295 hi-pressure pump, I have checked the clearance between the pan and pickup and it's at 3/8"s of an inch.

I am using a 20W50 mineral oil.

Not sure what else to try besides pulling the motor and checking oil clearances again.

I know for a fact that the plug was in on the front of the motor-if it's leaking I don't know. As for the rear, I installed a new gasket so I don't think anything is wrong there.

This is the second oil pump in the car, I pulled out a Mellings 296 and replaced it with the 295 with the red spring. Oil pressure increased at idle and cruise by about 15-20 across the rpm range-but at wot pressure was only 25-30.

When I moved the pickup up to 3/8" of inch off the pan from 1/4" pressure at wot increased by 10 psi. It almost looks like the pressure is trying to recover but cannot. As soon the car comes out of wot the pressure goes up immediately then drops back down then back up to around 40 psi.

I am told the clearance on the mains are .002 and on the rods .0025.

The crank was turned 10/10.

Before pulling the motor out I was thinking of just pulling out the Mellings and just trying the factory LS6 pump with a .040 though shim.

I have heard about cam bearings causing this but what should I be looking for here? Can they be changed without pulling the motor?

Another thing I have been told to check is crank end-play.

We have pulled the filter and cut it open-and it looks fine.

Just want to make sure everything is looked at before pulling the motor

Thx

 

I don't believe that the oil pump is the culprit, because changing to the 95 pump netted increased pressure, but did not change the pattern.
Was the oil pump o-ring mounted correctly, as in the photo? Conversely, if the o-ring is placed all the way onto the pickup tube, you'll have pressure troubles of varying degrees.


Do you know whether the rear cover was removed, and the oil galley barbell was replaced? If so, the barbell may be partially inserted, or it may have backed itself out, and could be unseating itself under higher engine speeds/pressure.

 

O ring is seated correctly...as far as the barbell backing out would that not cause an oil pressure problem all the time once it backed out for the first time

 

I had a similar problem to yours. After removing the back cover, I found the barbell unseated, with o-ring still appearing to seal, in the galley. It was able to be slid back & forth without much effort. My guess was that when the engine was hot, & oil under higher pressures, that oil would bypass the o-ring.

 

For your reference, my forged 383 iron block oil pressure never gets below 40 with M1 0w-40 oil, clearances are:

Mains: .0025 - .0031
Rods: .0024 - .0030
Cams: .0044 - .0067 (the high side is limited to one spot on the #5 bearing only)

I am also using the same oil pump as you and have the same pickup-pan clearance (~3/8"). Got almost 2k miles on it now.

To answer your question, NO you can not replace the cam bearings with the engine in the car.

I hope this helps you rule out some variables, however I would not pull the motor for only running 30 psi. Just keep driving it and watch to see if it progressively gets worse.