New Fuel Pump - PSI drops to zero with throttle


New Member
Oct 25, 2021
2003 Mustang GT 5-speed.

On Saturday I replaced my fuel pump from stock to a 340LPH Deatschwerks. After getting it in, the car starts and runs but really terribly. Put a code reader on it to watch fuel pressure in real time. It was idling at 13 PSI and if I give it some gas, it drops to zero. Eventually it conks out and I have to build pressure again and start it back up.

thinking it was maybe a bad pump, I swapped back to the pump that was in there (it was working just fine but I’m turboing the car) and the car did the same thing. 13-14 psi at start then drops to zero with throttle.

I did prime both pumps after installing them a good 10-15 times to work out any air and build pressure. Didn’t help.

Today I put the new Deatschwerks fuel pump back in and I also changed the fuel filter. The car did better today but it's still no good. When I started the car, the fuel pressure would go up to 38-40, then over the course of about a minute would drop and stabilize between 11 and 12 psi. If I primed the fuel pump before starting the car, the pressure would stay around 21 psi. I've seen that the fuel pressure is supposed to go up by about 10 PSI if you remove the vacuum line on the FRPS, but when removing the vacuum line, the car just dies immediately.

Put a volt meter on the FRPS with the car off. It gets 5 volts. When the car is on, the voltage drops with the fuel pressure. Starts out at about 2.7 volts, then when the car settles at 11-12 psi, the voltage sits at 1.3.

If anyone out there has ANY ideas at all, please let me know. I'm completely stumped as to what's happening considering it was working perfectly fine before I put the new fuel pump in. I have ordered a new fuel pump assembly (the bucket, float, and everything that goes in the tank. It should be here Wednesday and I'll throw that in and see what happens. I'm not sure if there's a crack or hole in the plastic line that sits in the tank that could be causing this. I didn't see one. Literally begging for help right now lol.
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When checking catalytic converters for being plugged with a vacuum gauge
If you have a plugged one and the vacuum fails to build under load
Sometimes if you have a fuel pressure gauge installed at the same time the fuel psi will fall rapidly
Not sure why this happens
Check your exhaust for being restricted or the cats plugged just for GP's
Sure looks like you possibly got the wrong pump
okay i did a little digging. here's my theory. your fuel pump is getting constant voltage supplied by the fuel pump driver circuit. when you gas the engine the injectors pulse more to feed the engines demand. your fuel pump isn't being given enough voltage to build the correct pressure, let alone sustain that at WOT, so the engine eats the fuel it has, runs out, and dies. 13psi sounds low to me but that might be normal for a returnless system at idle.

whatever you do DO NOT HOTWIRE THE PUMP. you will wash out your cylinder walls and destroy your engine.

you need to start from the pump and work backwards.

1. test the pump for voltage at idle then at WOT. there should be factory specs somewhere for your year.
2. then you do the same with your fuel pump driver module (its in the rear on the passenger side)

you should be able to jam a multimeter in there and do a voltage drop test. when you give it gas the pump should get more voltage to feed the returnless fuel system. i bet its not rising at the fuel pump driver module so you gotta do some testing to find out if the ecu isn't telling it to increase or what.

in short: check things in this order:
pump, fpdm, ccrm, ecu

if all of that is good then you might have a line obstruction. in the case I would pull it off the rail, disconnect it from the tank, and shoot some air in it to clear it out.

if the line is the clear and the electronics are good your car will run right.