1999 GMC Sierra Engine misfires

Tiny
YZRIDER647
  • MEMBER
  • 1999 GMC SIERRA
  • V8
  • 4WD
  • AUTOMATIC
  • 207,100 MILES
My truck misses under a load, does not Miss at idle, I have varified that it is the #8 cylinder that is miss firing. I have checked the fire from the plug and wire and both are good, fuel pressure is good and I switched the injectors and coil packs from a known good cylinder to the #8 cylinder and it continues to miss fire under a load, I also just replaced a bad fuel pump and filter, when I replaced these that's when this problem started, it ran fine before the fuel pump went bad.I am out of option and don't know what else it could be, thank you for any insight.

Eric
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Thursday, February 18th, 2010 AT 2:34 PM

6 Replies

Tiny
MERLIN2021
  • EXPERT
Have a look at the PCM ground at the back of right side cylinder head, there is a tech bulliten on it, I will email it to you.
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Thursday, February 18th, 2010 AT 3:18 PM
Tiny
YZRIDER647
  • MEMBER
Hello I wrote in and made a small donation and had the question:
Engine Performance problem
1999 GMC Sierra V8 Four Wheel Drive

Automatic 207100 miles

My truck

misses under a load, does not Miss at idle, I have varified that it is the #8 cylinder that is miss firing. I have checked the fire from the plug and wire and both are good, fuel pressure is good and I switched the injectors and coil packs from a known good cylinder to the #8 cylinder and it continues to miss fire under a load, I also just replaced a bad fuel pump and filter, when I replaced these that's when this problem started, it ran fine before the fuel pump went bad.I am out of option and don't know what else it could be, thank you for any insight.
And the solution gave was:

Have a look at the PCM ground at the back of right side cylinder head, there is a tech bulliten on it, I will email it to you

i recieved the bulliten and checked the pcm ground, it was a little dirty but nothing out of the ordinary, I cleaned it and put it back on and did not solve the problem, it still misses. Do you have any other ideas. The only thing I can think of is that there is a problem between the injector or coil pack and what ever controls them. Because like I said I switched a good coilpack and injector from a known good firing cylinder to the #8 which is miss firing and it still misfires. So im thinking whatever tells the injector to spray or the coil to fire is bad. But u guys are the pros. What do you think? Please any advice is greatly appriciated. Thank you

ERic
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Thursday, February 25th, 2010 AT 4:39 PM
Tiny
MERLIN2021
  • EXPERT
You may have a vacuum leak, get a can of spray gumout, spray the vac lines and maniflod area, when the idle changes, youve found the leak! You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak, repair the vacuum leak and see how it runs!
The other thing to be aware of is an engine needs three things to run, compression in all cylinders, fuel, and spark. Can you do a compression test on the engine? http://www.2carpros.com/car_repair_video/test_fuel_injection_pressure.htm
Go here and checkout our video.
http://www.2carpros.com/car_repair_video/test_engine_ignition_system.htm
and this one!
http://www.2carpros.com/dia/how_to_check_compression.htm
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Thursday, February 25th, 2010 AT 5:54 PM
Tiny
YZRIDER647
  • MEMBER
Ok ill try that, I did a compression check and all cylinders came back in specification
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Thursday, February 25th, 2010 AT 6:58 PM
Tiny
YZRIDER647
  • MEMBER
Well I tried spraying the gum out to check for vaccum leaks, there wasnt no change in the idle.I soak downt all the vaccum lines I could see.
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Friday, February 26th, 2010 AT 5:54 PM
Tiny
MERLIN2021
  • EXPERT
Check the PCM connector plugs, see if the injector is firing under load, do you have a scan tool? Freeze frame data could point to the problem, it will store info about conditions at the time of a misfire.
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Saturday, February 27th, 2010 AT 8:53 AM

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