When I did head gaskets in April I had this same problem, but the engine would not start. I had the "thump", but also had LPO oil pressure, but no start. I replaced the geo-rotor, regulator and the oil filter valves and inspected the front cover for wear. All was good. Still, no start. The ECM would claim that only 300 psi of HPO was being built, and I had similar LPO symptoms as the OP. I tested oil pressure at the temp sensor port in the oil filter housing.

I then replaced the HPOP and IPR, and air tested the entire system. No leaks. Still, I had low ICP.

I gave up after 4 weeks of troubleshooting at least 4 hours every day and tearing down the top end four separate times for inspection, towed it to a reputable diesel shop in my area. The issue ended up being a gray large plug near (under) the brake master cylinder that was not entirely plugged in. I did not believe them at first until they showed me where they had to zip-tie it to keep it from unplugging. They cut the zip ties, un-plugged and re-plugged the connector and the engine refused to start. They the zip-tied the connector tightly together and then the engine started and ran, and has run perfectly ever since. This could not be diagnosed without the use of a high end scanner, and even then, they struggles to find the problem. I have ELM327, but that wasn't good enough. The ECM wasn't tripping CEL and no soft codes were present. I have since replaced this connector.

I believe the OPs LPO "problem" isn't actually a problem, and people expect the system to work in a way that it actually doesn't. My LPO system has the correct psi at the correct RPM, even before the replacement of the parts. The level of oil in the housing when the engine is not running is key. If the drain valve is broken or leaking, it will cause a long crank. Everything else between the filter housing and the pump can and will lose its prime. I have validated this while having the engine half torn down and checking various areas for the presence of oil after cranking and letting the engine sit for a period afterwards.

EDIT: Also, even when pressure is present, don't expect the oil pressure gauge to move. Trust a mechanical gauge before trusting the dash cluster. I have an '05 with a rebuilt instrument cluster, rebuilt FICM and a new-ish ECM. I have NEVER seen my oil pressure gauge move while cranking. It has ALWAYS came up after the engine has fired.

 

 

 

I remember that one Thor, that was a genuine PITA, the forum is better off because you followed through with updates and didn't throw in the towel.

 

I forgot to mention something very important here.

I did find something that was causing my LPO system to pump slowly, initially when cranking and pick up speed after a couple of seconds. The upper oil pan to engine block gasket was pinched in such a way that it could have allowed the geo-rotor to pick up air. I replaced that gasket, the pickup tube gasket and oil pan gasket with the engine installed.

This being said, I did have oil bubbles in my oil filter housing initially, but they would go away after about 20 seconds of cranking. After replacing those gaskets, the bubbles stopped.

 

I'll snap a picture tomorrow.

 

Updates ?

 

Mark,
Unfortunately I was not able to work on truck over the weekend as I originally planned, - I pulled building permit for house addition on Friday and was busy digging foundations over the weekend. I did not expect for building permit to be ready past Friday, so I tough I will spend weekend working on car. That been said, as soon as I have a chance to do tests I will post results and video/pictures here.
Thanks for patience guys.