I'm chasing a weird fuel pressure issue at around 3/4 throttle where my pressure goes from 30-ish psi to 20psi instantaneously causing major stumbling/flat spot and then back to 30psi once I push through to WOT. It doesn't matter if I mash the gas or ease into it slow, the same thing happens. I can hold the rpms and the throttle in the flat spot for as long as I want, it never recovers until I let off or push though to WOT. I'm watching this happen on the fuel pressure gauge in real time as the issue occurs. I am now looking at this return/bypass block thing that I have no idea what purpose it serves or what it actually does. It's plumbed to the fuel pump, and the main return line from the throttle body dumps into it. I'm wondering if it has some check valve or bypass inside thats diverting fuel from the pump via the small "USCG" hose which is causing my major drop in fuel pressure in that certain RPM range.
The plumbing goes like this: Feed line - Line out from tank(new lines) into water fuel separator(brand new) into fuel pump(old but still seems to work and obviously still works at WOT) into fuel filter(brand new) into the throttle body.
Return line - out of the throttle body into the "block thingy" and then straight to the tank.
Small crossover USCG tube from the "in" end of the fuel pump into the top of the "block thingy"
Also it has been verified that the fuel pump is not experiencing any significant voltage drop during the entire RPM range. The fuel pump and fuel system setup/plumbing is identical to my previous engine which was a standard run of the mill 454 330 hp. The new motor is .030over, bigger cam, 10:1 compression. The new motor runs GREAT except in that one rpm area where the flat spot occurs.
How could a fuel system run great, then stumble and drop pressure significantly, and then pick back up and run great again??? It is running fairly rich (another issue to get tuned) and then goes pretty lean on the wide band gauge and then back to fairly rich.