I have a 2005 dodge cummins and I just finished a complete rebuild on engine. Replaced everything oil pump, oil cooler and went full high performance with coated bearings Hamilton cam and arp everything. Crank it up and runs good oil pressure then slowly drops as it warms up until it hits zero. Changed oil psi switch because I know they are a common issue and I do have a manual gauge on it because I know that the factory gauge is not true and it confirmed no oil psi. I changed the oil regulator plunger and spring in cooler housing and changed relief valve in housing also and still same problem. I am at a loss now. I did notice some wear in the journal where the plunger rides but didn't seem horrible to me. And when inspecting parts during rebuild noticed a good bit of wear on rockers but I didn't see that causing the issue. Also want to note that the reason I rebuilt engine was a loss of oil pressure while running the freeway cause number 6 rod bearing to spin so it seems as if the issue that ruined the engine may still be present I just don't know what it could be. Please help

 

So you have pressure when it's cold. So something is heating up expanding causing the loss of pressure?! Like a crack or?
Did you inspect the pick up tube by any chance?
You did put a new gasket on the pick up tube and YOU DID put the allen head bolts on the girdle where they belonged right?! The losing oil pressure causing the failure in the first place is very troubling! The oil pump would be my first guess!

 

Yes I inspected and cleaned everything new gasket on pickup tube and gorilla girdle put sealer on gasket for extra assurance. Pump is new but I have seen bad new parts before

 

What manual did you use to rebuild your motor? Seems there are no aftermarket manuals except those from Cummins that are about $200