There are three "check" valves in the engine. 1 is in the block, drivers side just behind the timing cover. This is a check valve that, when the engine is shut off and then cranked allows oil unfiltered directly from the LPOP to fill the HPOP reservoir and the left lifter gallery. There are two more valves in the rear oil cooler filter adapter casting. 1 is the pressure regulating valve and will dump excessive pressure to the pan. The second valve you can see with the filter removed and this is the filter bypass. If the pressure differential across the filter is too high the valve will open and bypass the filter to keep a good supply of oil to the engine and prevent the filter from blowing off the casting.
I have an aux feed to the HPOP reservoir from the oil filter adapter housing which helped a low pressure problem at high rpm. You might try something like this if you need more oil up there.
A piston cooling nozzle that has fallen off may create a LPOP low pressure complaint, but probably wouldn't have a drain back issue. You probably need to inspect the timing cover closely to see if there is excessive clearance there.
I have an aux feed to the HPOP reservoir from the oil filter adapter housing which helped a low pressure problem at high rpm. You might try something like this if you need more oil up there.
A piston cooling nozzle that has fallen off may create a LPOP low pressure complaint, but probably wouldn't have a drain back issue. You probably need to inspect the timing cover closely to see if there is excessive clearance there.
