7.4L low fuel pressure

[Papikev252]

So initially I had a P0300 code for a multiple random misfire so started with changing the spark plugs, fuel filter, wires, distributor, tested and cleaned injectors and replaced the fuel pressure regulator. I then narrowed it down to the reason im getting that code is because I have low fuel pressure. The specs call for 58-62 psi but I hooked up a fuel pressure gauge and it shows 48 psi while running and then holds 50psi when the key/engine is off. When I turn the key forward for the first 2 seconds and the pump primes it goes to 58psi but after it drops back down to 50. If I were to use a wire to jumó and keep the fuel pump going it’ll be a constant 58psi. This is also the specs I was getting with the old pump.


I've also replaced the pump as well. With the engine running and gauge hooked up I also pinched off the return line and the pressure will shoot up immediately and from what l've read is that that means the fuel pressure regulator is OK. Is it possible i put a bad pump? I have over 12v at the harness and have cleaned the ground connection. What other stuff can affect fuel pressure.

I also read this reply in another forum and want to get your opinion on the accuracy of it: “Thats normal, 55-62 is with vacuum not applied to the regulator. When idling with high vacuum a low of 45 PSI whould be normal. Fuel pressure is not your problem. Any codes stored in the ECM?”

 

[Spareparts]

Not running. Pinch off the return line. Turn on the pump for a second or two. Gauge should be 85PSI or more. If not wrong/bad fuel pump.
At idle the fuel pressure should be close to 58PSI. if not you have the wrong or bad FPR.
That is idling with the line not pinched.

 

[Papikev252]

Not running. Pinch off the return line. Turn on the pump for a second or two. Gauge should be 85PSI or more. If not wrong/bad fuel pump.
At idle the fuel pressure should be close to 58PSI. if not you have the wrong or bad FPR.
That is idling with the line not pinched.

Yeah it does shoot up that much with it pinched off. But at idle it’s 48. I think I’m gonna switch the FPR because it was a eBay one. I just thought if it held pressure long after engine is off, then the FPR was good. But that’s cheaper than getting another pump.

 

[Spareparts]

That might be but i think with key off bleeding down more means leaking injectors.
Someone recently within a day or two posted the proper FPR number. Here it is. FP10021

 

[Papikev252]

That might be but i think with key off bleeding down more means leaking injectors.
Someone recently within a day or two posted the proper FPR number. Here it is. FP10021

It’ll actually hold about 48 psi after the engine is off for about an hour or 2. I’m going to check it out again.

 

[Schurkey]

Download the service manual set for your vehicle.

Engine off, fuel pressure spec is given. Engine running fuel pressure spec is NOT given, but it'll be "lower" than the engine-off spec.

 

[Papikev252]

Download the service manual set for your vehicle.

Engine off, fuel pressure spec is given. Engine running fuel pressure spec is NOT given, but it'll be "lower" than the engine-off spec.

So it should be lower than the 56

 

[Schurkey]

Yes--if the pressure regulator is subject to manifold vacuum, like when the engine is running but not under heavy load.

 

[Papikev252]

Yes--if the pressure regulator is subject to manifold vacuum, like when the engine is running but not under heavy load.

Makes sense. I just put the Delphi FPR FP10021. Now when I key on it goes to 60psi then drops to 52. At idle it’s 52. But when I hit the throttle it goes to 56-57. So better than it was. I know for a fact the previous owner out a cheap eBay pump so in the future May change that to a good brand. But from what you’re saying it’s normal. I just need to figure out this P0300 code I’ve been dealing with. I thought maybe what I thought waslow pressure was it.

 

[Carlaisle]

Do you have the original injectors in it and the accompanying hot start issue that plagues 99% of these engines?