Corey Pifer

I have a stock 2002 GC laredo 4.0. It has progressively gotten harder to start and now will only start after cycle priming it(off-on-off-on), and it died while driving. So what I have done/tried. Fuel pump kicks on when key turned, fuel pressure gauge goes to about 5psi, then drops to 0, even when cycling the ignition. Swapped relays for fuel pump and asd with appropriate others, no change. Tested both relays with multi meter, even energized relays and tested, all with in normal range. When the key is turned to on the fuel pump kicks on for about 1/2 second them turns off, the relay does as well. I changed cpk, no changes. I pulled wiring connector from fuel pump and checked voltage, still kicked off after 1/2 second. Pulled PDC and checked all connections, tested all fuses for fuel pump, asd, dome, etc. Any further guidance/suggestions are appreciated as I'm seriously considering entering it in the rough truck and demo derby at this point lol

dave1123

The fuel pump is doing exactly what it's supposed to do, BUT the check valve isn't holding pressure. The engine is supposed to start on residual pressure and the pump relay doesn't energize until the PCM gets a CPS signal that the engine is cranking.

Here's the normal starting sequence. You turn the key on. The fuel pump charges the fuel rail with pressure, then shuts off. You begin cranking. The PCM gets a signal from the CPS telling it the engine is turning, then it energizes the ASD and fuel pump relays. The ASD relay controls both the ignition and fuel injector circuits. Engine starts. PCM questions the security system. Valid Key? Yes. Engine stays running. If not, the PCM turns off both the ASD and fuel pump relays. Engine dies.

I believe that check valve is on the fuel pump itself, but you can install an in-line check valve anywhere rather than pull the pump out. It may be in the filter, IDK. I DO know the pressure regulator is in the filter. You should show around 50 psi fuel pressure.