Low oil Pressure

[Trevalien]

ok i have had this truck for several years and have had the same problem that just keeps changing.
its a 1990 GMC Sierra K1500 5.7 TBI
At first the oil pressure wouldnt get above 15 psi even on start up i would drive truck sounded terrible and at idle pressure would be 3- 5 psi
I changed oil pump, rod bearings, fixed timing, and changed gaskets oil pressure regulated at 40 psi driving and would go down to 15 psi idle...it lasted 3 days at which point i started at 30 psi and dropped all the way down to 3-5 again
i heard some knocking pulled pushrods found them bent and one lifter collapse so i replaced them. i also replaced my oil pressure switch since everybody suppects it. Now when i start it i have 15 psi again and it drops to 1-3 psi. the engine runs better than before but the low oil pressure light comes on when i drive the pressure stays at exactly 15 psi the whole time until i stop then it drops again

my question is am i supposed to somehow "prime the sensor" and i just have air between the switch and oil thats giving it a false reading? ive adjusted the new valves twice and i had oil spewwing out of the rocker arms so i know there pressure

 

[Trevalien]

Update:
just increased oil viscosity to 20w 50, rebuilt TBI, and increased throttle, is like a bandage on an amputation but it keep my oil pressure good enough until i can fix the issue

 

[bdja]

Had the same issue with a different year. The screen from the oil pick up tube was clogged bad. I replaced the tube and no more problem. Also the o ring on the tube could be shot, thus you would be sucking some air instead of oil.

 

[Bill325]

Driving my just bought '89 GMC pickup home, the engine started running rough . Plus, the engine oil pressure dropped to an indicated zero at idle after the engine reached operating temperature. I stopped at an auto parts store, and bought a bottle of Chevron fuel system cleaner, and some 20w-50 motor oil . By the time I got home, the fuel system cleaner had done its job . Using 20W-50 oil solved the oil pressure problem . The truck had 220,000 hard miles on it , but the transmission had been rebuilt . The truck ran well for 3 years before the engine just gave up mechanically , but I got my $900 investment out of it !