I have a 1999 gmc sierra that I picked up last week with low pressure. 20psi cold and 3-5 hot idle confirmed with a manual gauge. I changed oil pump pickup tube o ring thinking that was possibly the problem but no luck. Fired it up and it was the same as before maybe a few pounds better but not much. I was worried I possibly pinched the o ring or it wasn’t seated completely so i put 10quarts of oil in the engine and jacked up the rear significantly to try and submerge the o ring and pickup tube to rule it out. Same result no change in oil pressure at all. Should I add more oil as the pickup tube might not be completely submerged yet or and I looking at bigger internal issues somewhere in this engine more than likely down the road. 5.3 vortec, runs great and is extremely quiet. Thank you guys

 

My Haynes manual says that you should be seeing 6psi at 1000rpm, 18psi at 2000rpm and 24psi at 4000rpm with a mechanical oil pressure gauge.

You say you changed the pickup tube o-ring? What about the oil pump itself? They say the main reasons for low oil pressure are:

1. Work out oil pump.
2. Bad bearings (need a rebuild?).

 

I did not change the oil pump or anything else. I was wondering if everyone thought that 10 quarts would be enough to cover the pickup oring to rule that out. Like I said I jacked the rear of the truck up significantly and there was no change. I’ve also been reading about cam bearings possibly being a cause as well

 

Well... changing the oil pump for a new one would be a lot easier than doing the bearings! I think I recall from looking at the manual yesterday, that you access it through the timing chain cover.