Ive been battling these weird fuel pressure problems since i switched from tpi to carb. Heres the scoop. Took out tpi pump and ran a hose down into the gas tank, ran a external pump under the car by the tank and an inline filter right after the pump. Used a mallory 3 port regulator before the carb.

The problem.. Fuel pressure jumps up to 7 psi on startup and within 5 min drops down to 0. I took the guage out when it was at 0 and there was just fuel laying in the line. Its not vapor lock because i ran a hose outside the car to the carb and did the same thing. Took the tank out today and ran the pump straight from a gas can with the return line in the can. That eliminates any problem in the tank. Tryed a used blue holley pump same results. Voltage at the pump is 12.xx. When the pressure drops to 0 the pump quits sucking and fuel was actually coming out of the line. Do these pumps go bad often? I think it is kinda weird that 2 different pumps would both be doing this.

 

What did you do with the charcoal canister?

 

just left the charcole canister there but isnt hooked up to the vacuum.

 

You probably aren't creating a huge vacuum in that short of time, but you might try with the gas cap loose/off.

Are you sure voltage is staying where it should be? How did you wire the pump?

Long shot, but it could be something like a collapsing line. I assume you didn't put a filter sock on the hose in the tank.

 

well today i dropped the tank and just ran the hose into a full gas can with the return line too. I didnt use a sock on the end of the hose and the voltage seems to be fine. The pump is wired through the oil pressure sending unit so it shuts off when there is no oil pressure. The pump just stops pumping after a few minutes of running. Shut the car off and start it back up and the pressure will jump up to 3-7psi depending on how long it sits, then drop again. Im going to go out and check the voltage after the pressure dops to 0 Any other ideas?

 

Update: Voltage stays the same at 12.9-13.0 whether at 7 psi or 0. I can feel the pump sucking my finger on the hose when the pressure is up but when its at 0 there is no suction. The weird thing is, after the prssure drops to 0, if you shut the car off and start it back up the pressure goes up and then drops back down again. Do i have 2 bad pumps? somethign in the wiring?

 

any ideas?

 

Ya I'm curious what this is too, its the freaking weirdest thing I've ever seen, We've eliminated just about everything, down to the bare minimum setup.

Right now, its hooked back up as line from tank, to pump, to filter, to regulator, regulator to carb and return from regulator to tank, regulator set at 7 psi. Sometimes when he starts it up, it will have a lack of pressure, but if he crawls under the car and pinches the rubber line in front of the pump, it will boost the pressure and then start pumping again. I've never seen so such a rediculous problem, and it's a huge pain in his, and mine, butt.