So recently I can swapped my 5.3 Silverado. It's a small cam. And everything is ok but some sensors are throwing codes. Anyways everything was fine and then today I put in A new fuel sending unit in it and after I finished putting it all in and connecting the new wiring harness I started the truck and it said check engine oil pressure and it read 0 on the gauge cluster. But once I rev it past 3,000 it picks up to about 25 psi and will run about 40 psi while I'm driving. But continues to read 0 psi when I start it until I rev it to 3,000 then it will go up to 25 psi. What could be causing this? Any help would be much appreciated

 

I know the oil sending units / oil pressure sensors go bad relatively often. I'd screw an honest to goodness mechanical gauge in there and verify that the oil pressure is indeed low.

And if it is, my money is on a oil pressure relieve valve or the oil pump.