2002 S4 2.7t with 044 pump using stock fuel lines. Car has been daily driven for a few years on a tune with zero idle and partial trims.

Recently I have been fighting with an extremely rich RKAT and RKAT2 (idle trims both banks) and slightly rich FRA and FRA2 (partial trims). The idle trims are enough to throw P1137 and P1139 codes (additive trims too rich for both banks) after a few days.

Ruled out injectors, o2 sensors, no vacuum leaks, no boost leaks, rebuilt diverter valves, replaced FPR, replaced fuel pump. We are at bad fuel return line or black check valve under the blue fuel tank hat.

I wanted to post here before I went ahead and replaced the return line and black check valve. Does anybody know if 30psi in the return line on a B5 S4 with 044 pump is normal? Seems way too high. Pressure in the feed line is 62psi at idle with vacuum line off and about 59 psi with vacuum line on which also seems too high. Vacuum on the car is good at about 22-23 in hg. Could the black check valve cause this?

Thoughts?