c5mtl

iTrader: (3)

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

wildcamaro

iTrader: (15)

30 psi is fine...you technically only need 10psi of oil pressure for every 1000rpms so in that sense you are fine...every LS engine I have had was between 30 and 40 psi hot...

c5mtl

iTrader: (3)

But I'm getting 35-40 at 6500

midnightbluS10

If you've checked any specs later than 1971, you'd see that those aren't very useful anymore.

for example a 2010 5.3L requires:

Oil Pressure - Minimum - Hot


41 kPa at 1,000 engine RPM 6 psig at 1,000 engine RPM

124 kPa at 2,000 engine RPM 18 psig at 2,000 engine RPM

165 kPa at 4,000 engine RPM 24 psig at 4,000 engine RPM

wildcamaro

iTrader: (15)

I got my specs from a GM service manual so....

ice95z

iTrader: (10)

IMHO your oil pressure is fine. The only thing I would make sure is that the oil isnt foaming giving false readings but I dont believe this is the case. Where do you want your pressure to be? The manual is a general guide line. Every engine is different and pressure will always be different from one engine to the next especially on a aftermarket build. If your pressure is below 20psi @ idle hot i would worry. If it was only 20-30 psi in the upper rpms then i would worry. To each thier own thats just my 2 cents with the past experience i have.

SPF77

The only input I have is for your pressure drop at WOT. If you want to maintain better pressure at 6500rpm I'd drop to using 10w-40 oil or getting a higher volume pan with good baffling. 20w-50 is just too thick to return to the bottom of the pan when that high volume pump is turning with the engine's 6500rpm. 30psi on a hot idle isn't bad though. I personally wouldn't be using 20w-50 unless my clearances were beyond .0025.

midnightbluS10

A service manual from 1971.

10psi for every 1000rpm is a "rule of thumb" for the old sbc. Prove that it says that in ANY service manual and I'll send you $100 cash. I ain't no f***** punk, either, so if you can prove it, you have my word that I'll send it.

BTW, I'm a GM service tech and I've NEVER seen that listed as specs for ANY vehicle in GM's SI. Or AllData for that matter.

Thanks

Justin Caskey
Shreveport, LA