Low Fuel Pressure/P0300 - 2003 GMC Yukon Denali

[Bxcliff]

I've been fighting with a p0300 (multiple cylinder misfire) code for a while so I had recently replaced my intake manifold gasket along with knock sensors w/ new harness, and while my fuel rail was out, I had the injectors cleaned by a professional with new o rings and filter for each one. The injectors came back and they were not that dirty but it was cleaned enough to make a difference. I had also replaced the FPR and after everything was done I decided to test fuel pressure for fun and I realized it was idling at 45 psi and would increase slightly when I gave it throttle. I have a Vortec 6.0l and I thought they were supposed to idle about 50-60 psi?

Other than the things I mentioned, I also replaced the fuel pump and the fuel filter about a few thousand miles ago. This p0300 code is driving me crazy; my next move is to replace the cats because they feel/sound like they're bad and it's been progressively getting worse. Any thoughts?

 

[afpj]

What is the fuel pressure with key on engine off? Multiple people have had problems with non OEM fuel pumps. What brand is the fuel pump? I don't think changing cats is going to help a bad fuel pump?

 

[Bxcliff]

What is the fuel pressure with key on engine off? Multiple people have had problems with non OEM fuel pumps. What brand is the fuel pump? I don't think changing cats is going to help a bad fuel pump?

The needle jumps straight to 45 psi when I put the ignition on and 55 psi when I pull the vacuum line off the FPR and rev it a little. I bought an Acdelco fuel pump, I think the part number was MU1321 but I'm not sure. I kind of wish I had tested the fuel pressure right after I installed it but I was just happy it was running again.

 

[OR VietVet]

If you do the cats replacements, do all O2 sensors as well.

 

[Fless]

The needle jumps straight to 45 psi when I put the ignition on and 55 psi when I pull the vacuum line off the FPR and rev it a little. I bought an Acdelco fuel pump, I think the part number was MU1321 but I'm not sure. I kind of wish I had tested the fuel pressure right after I installed it but I was just happy it was running again.

The initial test is without the engine running, to see what the fuel pressure ramps up to. Then see if it bleeds down quickly or holds somewhat steady, again, without the engine running.

 

[Bxcliff]

The initial test is without the engine running, to see what the fuel pressure ramps up to. Then see if it bleeds down quickly or holds somewhat steady, again, without the engine running.

With the key on and engine off, it held at 45 psi and it didn't go down at all

 

[Fless]

KOEO (key on, engine off) NBS non-Flex pressure spec is 55 to 62 psi; Flex Fuel is a little lower at 48-54 psi.

Are you getting any long crank times?

 

[Bxcliff]

KOEO (key on, engine off) NBS non-Flex pressure spec is 55 to 62 psi; Flex Fuel is a little lower at 48-54 psi.

Are you getting any long crank times?

Not at all, it starts right up and it always started the same way for years

 

[Fless]

That's a good sign. Have you looked at your short- and long-term fuel trims?

 

[Bxcliff]

That's a good sign. Have you looked at your short- and long-term fuel trims?

I have and if I'm being honest I know nothing about that type of stuff. I'll borrow a scanner and do some research on reading fuel trims and check it out. What would the fuel trim values need to be for it to be running properly?

Also, I had a buddy install my new cats, the truck drives a lot smoother and there is no more restrictions but it still has the p0300 code and misfires with a rough idle. I'm starting to think it may be a bad lifter.