Just purchased a 2008 Silverado 5.3.
this is probably the 20th 99+ gm v8 I've owned, and I've done work on several more but this one seems strange to me.

truck had good pressure, then it randomly dropped to 0. turned the truck off for 30 seconds, restart it has 40psi for about 20-30 seconds then slowly drops to 0. Also dosnt fluctuat with throttle . Any ideas? Screen, sending unit, o ring?

It does have a lifter tick, but i dont believe that to be the complete culprit, or any thing to do with the pressure issue, no other nocks or any noise changes from 40psi to 0 psi

 

I'd look at the sending unit. Replace the screen while you are there.

 

Any simple place to get a mechanical gauge on? Oil galley plug by the power steering pump? If so any one know the thread pitch/size?

 

I had the exact problem with a bad sending unit....and it was only a few months old

 

On a gen iv thats not a true test of pressure as they still have another relief valve in the oilpan threaded above the filter. I just have the gm adapters that screw on in place of the oil filter which will show the same pressure as that gallery plug does. When I tested my 6.2 (no afm obviously) I had close to 20 psi differential from what the scanner reads off the sender to my test gauge. My test gauge is an annually calibrated $5000 deadweight so its accurate lol. I swapped the sending unit on the truck and there was zero change. I'm sure if I pull off the oil pan and remove that stupid relief valve then the gauge would match the test unit..

If it's a gen 3 set up it will be accurate to what the sending unit and the engine sees minus any loss from the oil filter.

 

First change the oil with a good filter, just helped chase down a similar issue and the filter o-ring blew out. Cheap filter failed and the simplest solution was the right fix after he dropped a grand at a dealer for "diagnostics". My 08 Tahoe started the lifter tick but just needed the correct weight oil and fresh filter after about 5k miles. Both were throwing codes for Oil Pressure Sender and his threw a code for something these trucks don't even have. Both fixed for a whopping total of $50 each.

Next verify true oil pressure or just change out the sender and clean the screen. You can move some stuff around and change the sender without pulling the manifold. Its annoying but can be done in a couple hours with the right set of tools.

Good luck

 

Recap: new screen and sender fixed it kind of. It done it once more after the change (about 10 miles after) restarted the truck and hasnt done it since (200-300 miles) but now its throwing a code for the oil pressure switch along with service stabilatrac and traction control but gauge dosnt go below 20psi any more.

 

I had a similar issue to this, but it turned out to be not an issue with the sender or screen. I'll post up my own thread on it. Glad to hear you got it going though.