I installed a real and brand new walbro 255 gss342 fuel pump about 2 months and a half ago.
As mentioned in other thread, about 1 month and a half (maybe a bit more) I suddenly started to have strange fuel trims (although fuel trims behavior goes in the opposite direction of the potential cause of the problem).
2 weeks ago I checked fuel pressure and it was ok at idle and at part load / full load (about 2,5bar at idle and 3bar at load with stock FPR and 3,5bar at idle and 4bar at load with 4bar FPR).
At the time I didn't go for a run to test the fuel pressure while driving and pulling but with the car stopped and reving it I could test fuel pressure under load, almost as if I was driving - this is because the car builds a lot of boost even stopped due to the stronger WG spring.
However, with the engine off and key on position II (priming the system) the fuel pressure is really low. I don't recall the number exactly but it was about 10psi or even less. The system should hold about 3bar or 4bar (depending on the FPR I had installed) with engine off and key II.
Once the engine starts the pressure goes right up from the 10psi but as soon as it turns off the pressure drops a lot and quickly (I don't remember very well but I think the pressure dropped almost to 0 - less to 10psi was certain).
At that point (and once I had already replaced the FPR) I immediately suspected about a bad fuel pump check valve.
I've been also noticing the car takes a little bit more to start (a few more cranks than before).
Furthermore, I get almost no knock if I pull a gear from 3000 to 6500rpm for example but as soon as I change the gear quickly (like from 2nd to 3rd) and floor it again I get significant knock retard. In one of my logs I could see that between the shifts AFR went really lean (19AFR).
Its seems that when pulling the fuel pressure stays high and good as it should be but when changing gears (where absolute fuel pressure drops) it probably drops more than should and/or it takes a bit more time to go high again - probably due to a bad check valve not allowing to strongly hold the fuel.
Has anyone else experienced something like this and/or have had bad check valves on walbro pumps?
I'm pretty sure all of this is being caused by the check valve. Can't see what else can cause this because I've replaced FPR, checked vacuum, injectors don't leak, evap seems ok...