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.

 

[OR VietVet]

Typically, if you had a bad lifter, you would hear that lifter(s). Plus, I doubt you have 1 bad lifter causing a P0300 code.

Here is a link about fuel trims: https://www.autoserviceprofessional...-works-and-how-to-make-it-work-for-you?Page=2

 

[Fless]

Typically, if you had a bad lifter, you would hear that lifter(s). Plus, I doubt you have 1 bad lifter causing a P0300 code.

Here is a link about fuel trims: https://www.autoserviceprofessional...-works-and-how-to-make-it-work-for-you?Page=2

Beat me to it, @PNW VietVet! (sound familiar?)

That's a good read, @Bxcliff, so take note of the tests at a couple of RPMs. Trim issues may or may not cause the misfire code but it should be easy enough to rule them out as a cause. Post up both LT & STFTs on both banks at both RPMs, and let's see what it's doing.

Your STFTs won't start to change until it's in closed loop, after the engine is running for a minute or two. Until then they'll be '0'. In other words, there is no instant fuel trim while in open loop. Those O2 sensors need to warm up to start working.

 

[Bxcliff]

@Fless
@PNW VietVet
Thanks guys, I'll give it a shot and keep you guys updated. And I'm sure the lifters are going bad also not just one, I did some research and there was always a ticking sound that got gradually worse.

At first, it would go away once the truck was warm but now it sounds like a tractor all the time. Also, since I replaced the cats and it's not restricted anymore so when I'm WOT it backfires at a higher RPM.

This is really weird and it's driving me crazy

 

[Fless]

@Fless
... there was always a ticking sound that got gradually worse.

At first, it would go away once the truck was warm but now it sounds like a tractor all the time.

[COLOR=#ffffff]That is a symptom of broken or missing exhaust manifold bolts. Might want to see if they're all there.[/COLOR]

 

[Bxcliff]

[COLOR=#ffffff]That is a symptom of broken or missing exhaust manifold bolts. Might want to see if they're all there.[/COLOR]

Not the case with me, I replaced my manifolds and used ARP bolts last year with headers and wrapped them with kevlar heat wrap.

 

[Bxcliff]

So I put together this video of all my fuel trim data in this video, check it out and tell me what y'all think. I recorded it at idle and revved around 2k rpm which is where the CEL will flash for p0300 as shown at the end.

 

[Bxcliff]

Update: Iv'e done more research and a buddy of mine has a Snap-on scanner that could relearn both the crank and cam position sensors; I'm going to give that a try. What do you think of the fuel trims, @Fless

 

[wildcatgoal]

Bad cats are not very hard to detect. Check out South Main Auto on YouTube and search for cat videos for some examples.

I have successfully diagnosed bad cat converters on 3 cars now after those cars had had new O2 sensors, seals, pumps... the parts cannon. Everyone wants to avoid the cats because they cost a g'dam fortune.

The way I did so was:
- Car #1: stab to the floor revs of the engine while in park felt and sounded choked. Further testing on a cold morning demonstrated the amount of exhaust you'd expect doing so was not exiting the tail pipe.
- Car #2: temp laser after driving before and after the cat. The temp before the cat was hotter than after the cat on one side of the car. Replaced both cats for good measure.
- Car #3: car had what sounded like valve train noise and was difficult to start. Owner attempted oil additives, no joy. Discovered the owner added ZDDP additive to the oil at every oil change and did mostly stop and go driving on short trips. That'll block up the cats. Replaced, no noise.

 

[TheWhiteGhost]

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?

Hey Fless, I'm having the same issue:

key "on" 54.5 psi
Engine "on" 52 psi
After engine "off" 56 psi
after about 5 min slowly dropped 46 psi it might have kept dropping but i discounted it.