low oil pressure

[halofreek]

I just bought and put in a rebuilt 2.9L v6 engine in my 1989 b2 4x4. The engine is bored over .0040". There was about 500 miles on it when I installed a mechanical gauge I noticed the oil pressure was really low. On start up it would jump up to 55-60psi but after it warms up it runs at 0-5 psi at idle and about 30psi at 2k RPM. I installed the melling 328 high volume oil pump and it didn't change at all. I am using 5w-30 with a stp filter. Any suggestions as to what could be the problem could be?

 

[RonD]

Do you get lifter/valve train tapping noise at idle after engine is warmed up?
If not I would try another mechanical gauge or check the gauge tube for air.
Air in the line would give false low reading at low RPM as it absorbs pressure more than oil, similar to spongy brakes when you have air in the lines.

Under 3psi should give you tapping noises.

Rule of thumb on oil pressure is 10psi per 1,000RPM.
So 25-30psi at 2k RPM is OK
At idle, 600-700 RPM, there is only 6-10psi

Oil travels from the pan to the pump and then to the filter, from the filter it goes to the main passage, which is where oil pressure is measured, and then to the bearings and lifters(valve train).
Oil pressure is the back pressure from the volume of oil being pushed through the bearings and lifters.
Common cause of low oil pressure is worn bearings that allow more oil to pass, and this happens as engine gets older/more miles, but shouldn't be happening on a rebuild.
The oil filter can restrict volume which would reduce back pressure, high flow filter might be worth a try.
Cracked pickup tube for the pump can reduce the volume going to the filter as its suction is reduced.
High volume pump may be pulling too much oil out of the pan, so same as cracked pickup tube, it loses suction as oil level drops, this is a long shot, lol.
Check oil level when cold then after running the engine for awhile shut it down and then check oil level.

 

[wildbill23c]

I just installed a manual gauge a couple weeks ago in my 84 B2 with 2.8L and have extremely low oil pressure according to the gauge, but if in reality it was as low as the gauge reads I would have damaged the engine by now. Either the gauge is faulty, there's a leak in the connection or air in the line. Mine reads fine at start up around 40psi, but as it warms up the pressure on the gauge drops to around 0-5psi as well, I haven't taken the time to find the leak which I'm sure is what the problem is, just the connections on the line aren't sealed correctly. As the oil thins as it gets warmed up it will leak worse causing the extremely low pressure levels, also if you have any severe oil leaks on the engine itself the pressure can be low as well.

 

[bfiakas]

Try running with 10-30W oil if your areas temps are in the 20s or above.You should gain 4 psi across your RPM range.

 

[bfiakas]

Do you have lifter noise or rod knock on start up?