The Stovebolt Site Forums The Shop Area Engine Shop low oil pressure

I have a 250 in a 55 GMC. Due to previous low oil pressure I replaced rod and main bearings and oil pump. At start up oil press. is 40 psi. After about 20 min. the hot idle press. is near 0 psi. At road rpm the press. is up to 30 psi. All bearing clearances are .0015 min. Any ideas on why this is happening? The engine runs smooth with absolutely no noise. Thanks. :confused:

Cam bearings are shot.

Sounds like Chip is right. Have you pulled the valve cover to check for oil flow to the valve train?

bmc1: You did the right thing first (mains) Chip and Fred also on right track. Sending Unit Ok does it get noisy when it drops oil pressure? popcorn The Truth is Out There (Good luck)

Plenty of oil up on top. Verified pressure with two mech. gauges. Engine at idle is smooth and quiet.

You may be able to go to a thicker oil to limp it along, but it sounds like you need to check cam bearings and oil pump relief spring.

Thanks for all your help. Running 10w30 now. I have switched to 20w50 in the past to gain about 10 psi. What do you guys think of oil additives such as sick 50 or greased lightning for this problem??

After reading the following article, I place little value in the "miracle cures".
Old Engine FAQ on oil additives

I don't believe in miracle cures either. I must still be delirious from spending the weekend changing out the bearings. Thanks for your help and advice.

Did you just get the truck or engine? Was something changed that might have started the difference? That out of the way here's some unlikely things to do while your pondering pulling the cam...
1) Changed the filter? Maybe something goofy with it's bypass (not sure if this would cause drop in pressure though).
2) Replace the spring as Chiphead mentioned.
3) Take a sample of oil and test the temperature.

I would suggest that since the pressure is much higher when the oil is cold, this has nothing to do with the relief spring. This is most likely a clearance problem somewhere.

Just drive it, will probably last forever in normal use.Cover the guage with a piece of paper and draw a guage face with the needle on 50 psi.

Tony's comment about just drive it reminded me of a similar situation years back. On our fire dept we had a 54 Reo 6X6 with the 331 c.i. six. Showed oil pressure when cold, then dropped to zero. Local mechanic diagnosed it as worn cam bearings and said to just drive it, it won't hurt anything. We drove her for several years like that, never had any problems.

I turned my idle up a little. A slightly faster idle will up the pressure a little. Mine is low at idle when I get off the freeway with hot oil. Next I will change from the multi grade to straight 30w oil. The engine runs excellent and there are no noises.