2004 Mach 1 losing fuel pressure. Bad fuel pump?


May 23, 2022
Hello everyone,

Here’s my issue and some backstory on why I started troubleshooting. So my 2004 Mach 1 will never start the first try. But it will start the second. I’ve changed the battery, I needed to anyways so I’m not sweating over the battery change. The fuel pump is new. It’s a Walbro 255LPH. New doesn’t mean good. So in another thread I posted I was chasing down a crank no start on this car and finally fixed it. I changed the fuel pump in the process. That’s when the hard start issue came about. The car runs great and I love driving it. But I want to tackle the hard start. So I plugged in my fuel pressure gauge and in the run position, it reads 5 PSI. That’s criminally low lol. I can cycle the key from off to run about 4 times and it will build up to about 35 PSI which is its normal range of fuel pressure. But between 30-40 PSI, the fuel pressure will drop to 20 PSI within one minute. It will fall to 10 PSI and stabilize there and it will take about half an hour to drop any further. When the car is running, it is about 30-35 PSI at idle. As soon as you turn the car off, it again drops QUICKLY to 20 PSI, then down to 10 PSI and stops dropping. What do you guys think? Fuel pump? It seems weak to only generate 5 PSI in the first cycle of off to run and I’ve gathered there are check valves inside the fuel pump itself that can go bad. There are no fuel leaks I can see, as in no fuel on the ground, around the injectors, etc. I also suspect a leaking injector but I doubt the fuel pressure would fall THAT FAST if a injector was leaking. The fuel filter has been changed when I put the Walbro pump in a couple months ago. What do you guys think? Fuel pump? I’m open to all advice here. There are no check engine lights.


Active Member
Jul 12, 2018
Since the issue occurred when the pump was replaced, it sounds as though the anomaly originated there. There’s no ‘ it’s this’ guarantee, but elimination process until issue(s) present, works! I’ll start slow, a few tests..
-Check voltage across terminals during priming-
- Check for solid grounds-
-odor of fuel in oil, wipe a bit off the filler-
1) Pump malfunction, leaking pressurized fuel back into tank.
2) Weak Grounding or other compromised electrical connections connected to the pump, voltage drop, disturbed when work was performed.
3) Leaky injector(s) can drop pressure quickly upon shutdown, really doesn’t take much.
4) reg.
5) Relay responsible for priming, contacts, connections..
Good luck!