Oh, I’ve been through this whole thing. I’m not interested in cutting up the car, so I’ve removed the exhaust (and everything else in the way back there) and dropped the tank out twice now.
Basically the key on pressure is low, and drops back to zero after a few seconds. Car does start with a longer crank and runs at the correct pressure once started. I think it’s the pump itself, which is a new Delphi… I found a 3 way fitting in the gauge kit to go on the pressure line at the fuel filter. I tested the new fuel pressure regulator first (clamped the return line, still lost pressure fairly quickly after cycling the pump).
Then I tested to see if the check valve is allowing fuel to drain back into the tank. Clamped the pressure line before the fuel rail, cycled the pump, and got the same result. Methodology is correct here right? If the fuel is just in the pressure line, isn’t able to go anywhere else, and it’s losing pressure then it has to be draining backwards after the pump shuts off?