any help for diagnosing my misfire??..

[t3xas_m4de]

Ive got a 2000 gmc sierra 1500 5.3 ive got 210xxx miles all stock minus headers and egr delete. Ive got a real bad misfire on cylinder #1 and #8. It comes and goes it is worse at idle or low speeds, at 70 mph the mis will go away.

 

I have 2 codes on the truck one egr code and a p0300. Egr has been deleted for 6-7 months, i havent had it tuned yet to turn off egr, but i know this is another problem. It only had a slight mis when i first deleted the egr.

 

I have swapped coils and wires on 1 and 3. Didnt change it. Swapped injectors on 1 and 3 didnt change it. Ive got a new MAF brand new ACdelco fuel pump, new fuel filter and spark plugs were changed a little over a year ago. Ive also sprayed carb cleaner all over the engine thinking maybe i had a vaccum leak, it didnt change idle any. Im stuck and getting tired of throwing parts at it. Any suggestions?

Ive also checked and changed fuel pressure regulator, with no change..

[newdude]

Intake manifold gaskets? Any other codes besides the P0300 or the EGR?

[t3xas_m4de]

No other codes whatsoever. I was told if the intake gaskets were leaking spraying carb cleaner around them should make it change idle,i tried it with no change. The thing that gets me is sometimes it runs fine sometimes it barley wants to move.

[sefiroxx]

Vacuum leak usually causes a lean condition code.

 

If it barely moves - does the engine rpm barely change?

 

P0300 is a generic misfire code, did you get 00301 and 00308 codes (need a better code reader).

 

When it barely moves, put in neutral, pull air tube, check throttle movement, spray starting fluid (carb) to see if idle change - if yes, then fuel system. Check with fuel pressure gauge.

 

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[t3xas_m4de]

Vacuum leak usually causes a lean condition code.

If it barely moves - does the engine rpm barely change?

P0300 is a generic misfire code, did you get 00301 and 00308 codes (need a better code reader).

When it barely moves, put in neutral, pull air tube, check throttle movement, spray starting fluid (carb) to see if idle change - if yes, then fuel system. Check with fuel pressure gauge.

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Only codes stored are: po300 and p0405

The reason i know its missing on 1 and 8 is my snap-on scanner has a misfire data selection, i dont think a better scanner would tell me more.

When i say the truck barley runs, i mean it just misses on every revolution it feels like, sputters and puts like a v8 running on 6 cylinders haha, but the problem comes and goes, seems to be worse when its warm or its been driven for a while.

 

By air tube, do you mean the one with the pcv valve or what??

 

I will put it on a fuel pressure test and see what it says.

[t3xas_m4de]

I have a fuel pressure gauge on it now, at idle it has 41-42 psi. What is normal for these engines?

Whenever i unplug my fuel pressure regulator it jumps to 49-50psi. Is this my problem?

[t3xas_m4de]

Cleaned ground for fuel pump double checked connection at pump. It idles at 45 psi now. Still missing on 1 and 8.

[Dnt]

Do a compression test on all cylinders and report back.

[t3xas_m4de]

Do a compression test on all cylinders and report back.

Well i could kick myself right now.. i went to do my compression tests this morning, started on cylinder number one, pulled the plug and there were two cracks on the porcelain... i completely overlooked them since the problem started after a fuel pump failure, and i had changed plugs within the past year or so. Keep it simple stupid...

Thanks for all the help guys, its much appreciated!!!