Hello I have a 1967 Pontiac Firebird with a 400 motor. For the life of me I cant figure this out. I have a Edelbrock 560 cfm carburetor with a Holley mighty might electric fuel pump 4-7 psi range. I have a Edelbrock fuel pressure regulator with a return back to the tank along with a vented gas cap.

OK here is the problem when I start the car the fuel pressure on both the regulator and the one on the carburetor fuel rail going to the carburetor is a 4PSI. But after driving the car after maybe 30 minutes BOTH gauges read down to 0 PSI including at idle. The car runs fine it not overheating no surging no stalling. I looked at the plugs very carbon fouled and i was using AC Delco irredieum plugs.

I initially though that the fuel pump was too big for this car it had a Mallory 4140 fuel pump so I bough a Holley mighty might fuel pump still having same issues as listed above. Then I bought a Edelbrock return type regulator due to the fact the regulator I had on there was a deadhead type regulator (no Return Port) still having the same problem. Then I rerouted the fuel lines along the frame rail away from exhaust with 3/8 inch rubber lines and also dropped the fuel tank and bought a 1969 fuel sender that has the supply and vent line attached to it. Be advised that my fuel tank has a sump welded on the bottom of it and goes from the bottom of the fuel tank to a fuel filter then to the fuel pump then to the regulator and the return fuel line goes back to the fuel tank via the original 69 fuel sending unit and i did cut the fuel supply screen mesh screen off so it does not have any restrictions when the fuel return back to the tank. the other line with is the vent line is capped. I tried to take the gas cap off and see if the pressure return to normal after a few minutes but no avail.

I keep thinking I am having a c aviation issue but the Holley fuel pump is small and the vented cap should be sufficient for venting. so my question is this normal for the pressure to drop to zero on both my gauges or should they remain a 4 psi all the time while the car is running. I also check with I open the throttle I am getting plenty of gas from both the primary and secondary jets. Any suggestions please.