I know, been covered 1000's times PO300B code

[Frosted Flakes]

I know this subject has been beat to death and I have searched thru the archives, but nothing has helped. I am at my wits end with this problem!!!!!

 

So here goes, I have 2004 GMC 2500HD with 6.0l and have the misfire code popping up when the truck reached over 2600rpms and going roughly 65-70 mph and the lights flashing like a big dog. The poor truck is screaming at me to stop it, but the speed limit now around here are 70 and I like to get where I am going fast! Imagine that!

 

What I have done so far to evlivate this code is:

 

Put new plugs and wires on…..plugs gapped at .040

New coils for cylinders 1,5,6, and 8

Fixed the exhaust leak on the driver side manifold. The number 7 cylinder back bolt sheared off, imagine that! It really helped that I am welder by trade, and still was a major PIA to get out

Checked fuel pressure…..60psi at the rail and ran it hard on the tollway, never flickered

Fuel pressure held at 55psi when truck was turned off, held there for more than 10 mins

 

So I got a scanner and grabbed screen shots to post here for you guys to mull over and throw me your 2 cents

 

Starting with the misfire data

 

 

 

 

As you can see 1,5,6,8 are missing with 5 and 8 are really out of whack!!

[Frosted Flakes]

Here is Engine Data link 1, I think thats what the heading said on the scanner

 

 

 

[Frosted Flakes]

Engine Data Link 2

 

 

 

[Frosted Flakes]

This was the last screen shot of the misfire history before shutting off the truck

 

 

 

 

 

With all the information I hope someone could lead me in the right direction of whats going on. Also, the truck never fails to start within the first 3 seconds of starting, truck doesn't hesitate when you put your foot in it, and it is smooth as silk when running, no rough idles, no lack of power, nothing seems out of the norm for it. Just crazy!!!!! The check engine light only flashes when above 2500rpms and going 65-70mph. Before I went out to the tollway, I did some hard acceleration around the house in the neighborhood of 3500rpms from a dead stop. She pulled like there was nothing wrong and the check engine light did not come on. It only comes on at steady speed and rpm.

 

Any help would be greatly appreciated!!!

[Jsdirt]

Does that engine have AFM? Seems like only 4 cylinders have the misfire problem - if it has AFM, it could be a lifter issue, but I don't recall AFM being around in '04 - can't remember one way or the other.

 

Check the usual suspects - ignition components of course, and intake manifold gaskets. Spray the intake gasket area with brake or carb cleaner and note any RPM changes. If all that checks out, next step for me would be a compression test.

 

What is the "B" at the end of the code? Never seen that before ...

[txab]

Checked intake manifold gaskets? If they haven't been replaced, they may be due

[Frosted Flakes]

Does that engine have AFM? Seems like only 4 cylinders have the misfire problem - if it has AFM, it could be a lifter issue, but I don't recall AFM being around in '04 - can't remember one way or the other.

 

Check the usual suspects - ignition components of course, and intake manifold gaskets. Spray the intake gasket area with brake or carb cleaner and note any RPM changes. If all that checks out, next step for me would be a compression test.

 

What is the "B" at the end of the code? Never seen that before ...

 

Okay, you got to remember, I am completely illiterate to the acronyms! What does AFM mean? As for the B, thats what the scanner spit out to me for the code.

 

Just to understand the thinking, why do you think 4 is the problem? 5 and 8 are crazy in the numbers for misfires, is it because the 4 cylinder had the AFM?

 

What would the compression tell me? Worn rings?

 

I did forget to mention the truck only has 109,000 miles, she's pretty much my snowmobile hauler or whenever I feel like taking it versus the welding truck.

[Frosted Flakes]

Checked intake manifold gaskets? If they haven't been replaced, they may be due

The thinking behind the manifold leak is its leaning out the cylinders misfiring and thats messing with what? The O2 sensor downstream before and after the cat convertor?

 

I have not replaced the gaskets, so that will be the next step tomorrow. What about bad injectors? I know I didn't get a code for any of them, but could it so slight that the computer doesn't pick them up?

 

The thing thats been driving me nuts, is that the code is only created at higher rpms. I drove around under 55 and nothing, its till I hit 65 or 2600 rpms. Is that when the vacuum is created the most in the intake manifold?

 

Sorry, for so many questions, I just want to get this fixed, I'm heading up north sledding next weekend and I want to understand.

[Jsdirt]

 

Okay, you got to remember, I am completely illiterate to the acronyms! What does AFM mean? As for the B, thats what the scanner spit out to me for the code.

 

Just to understand the thinking, why do you think 4 is the problem? 5 and 8 are crazy in the numbers for misfires, is it because the 4 cylinder had the AFM?

 

What would the compression tell me? Worn rings?

 

I did forget to mention the truck only has 109,000 miles, she's pretty much my snowmobile hauler or whenever I feel like taking it versus the welding truck.

 

AFM = Active Fuel Management. That's the 4-cylinder mode on a V8 at light load cruise. It uses different lifters and cam profiles on the 4 cylinders that get deactivated.

 

System seems to be nothing but trouble - lots of oil burners, and failed lifters and cams. Compression check would rule that out, if it's even equipped with that. If the valves are either hanging open, or not opening far enough, that would show on the compression test. A leak down test would tell you for sure what the issue is - valves, head gasket, loose spark plug (not common .. but it does happen when techs get sidetracked), or rings.

[Frosted Flakes]

Here is a question, why is the loop closed? I thought the loop was only closed when the truck is warming up. The truck was at 195-200 degrees, wouldn't the loop be open to have the sensors telling the PCM whats going on?

 

I just read something about the loop, but do not have any idea if this is the same loop they were talking about.

[Jsdirt]

Open loop means the computer is running off pre-programmed info - the sensors are not in play. This is during warm-up.

 

Closed loop is when all the sensors are active and communicating with the computer for proper air/fuel mix.

[jhplak325]

Okay, you got to remember, I am completely illiterate to the acronyms! What does AFM mean? As for the B, thats what the scanner spit out to me for the code.

 

Just to understand the thinking, why do you think 4 is the problem? 5 and 8 are crazy in the numbers for misfires, is it because the 4 cylinder had the AFM?

 

What would the compression tell me? Worn rings?

 

I did forget to mention the truck only has 109,000 miles, she's pretty much my snowmobile hauler or whenever I feel like taking it versus the welding truck.

Your truck does not have AFM since it is an HD and AFM did not show up in the trucks until 07.5 in the 5.3's

[Jsdirt]

That's what I was thinking - thanks for confirming.

[efritz]

Checked intake manifold gaskets? If they haven't been replaced, they may be due

 

LISTEN TO HIM ^^^^^^^^^^!!!! Everyone replaces a thousand parts just to find out its the intake manifold gaskets! easy job overall if you have some mechanical ability. do the knock sensors while you're in there. For these two things use ONLY GM PARTS or you will be doing it again....

[Jsdirt]

I think Fel-Pro makes a better gasket than GM does, although they are similar in construction.