Ok let me start out like this I am new to this particular Forum and iI am hopping and praying that one of you can help me out because nobody else seems to know what this issue is.
Ii bought a 1995 motorhome it is a superduty chassis with a port injected 460, this was sitting for about 6 years. After the engine warms up it will not accelerate it will start popping and surging it runs good until warm.
Now here is what I have done and or tried so far.
Dropped fuel tank cleaned installed a new fuel pump and filter (this is the new style Ford 4 wire pump).
Installed new cap and rotor.
Dropped tank second time installled a second new pump.
Checked fuel pressure 30-32 psi at idle 40 psi at WOT cold, 30-32 psi at idle and jumps to 40 at WOT but then falls off rather quickly to 20-25 psi (jumps around) and start to pop and backfire when warmed up. if i unplug the fuel pump relay for even a fraction of a second it will run perfect for about 5- to 10 minutes over and over again. If I disconnect the vacuume line to the regulator the pressure goes to 40 psi and remains steady, there is no fuel leaking from regulator.
Fuel pressure with pump blocked off is around 90 to 100 psi.

Fuel tank seems to build up a lot of pressure (80 gallon steel tank) and yesterday while I was trying to figure this out I relaesed this pressure by taking off the gas cap and afterwards engine would barly run off idle. The fuel pressure was around 30 psi at idle but as soon as I cracked the throttle at all the pressure would fall off. I had noticed earlier that when I disconnected the fuel filter the gas continued to come out even after the filter was empty so I guess that the pressure in the tank was pushing out fuel even with the pump off.
Is it possible that the pump is cavitating while the tank is pressurized and dissconnecting the fuel pump and reconnecting the pump restores flow????
Why wont is run when warm with no pressure in the tank?
Someone told me a stuck injector is the cause of the fuel pressure dropping but then why would dissconnecting the fuel pump for a second make a differance or un-stick the injector???
Any help or sugestions would be greatly appreciated.
Wes

 

Just a guess, but it sounds like your fuel tank vent is FUBAR.