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2002 GMC Denali is throwing a PO300 code after replacing Plugs and Wires

Tiny
OLDHAMMER47
  • MEMBER
  • 2002 GMC YUKON
  • 175,000 MILES
Thought it was a good Idea to go ahead and replace plugs and wires but now I dont know so much. I have a 6.0 with 175,000 miles. About 2 months ago stopped by the NAPA store and got new plugs and wires. They gave me the Iridium pugs and Standard OEM wires. The Iridium Plugs I know needed to be gap at.40mm and we sat right there at the counter and checked everyone. I didnt have this code prior to installing the new plugs and wires. About a have a day later my wife told me that the service engine light went off and I drove it down to the Parts place and they told me it was a PO300 code Random Misfire. I should NOTE that the truck is running GREAT! Idle and hard acceleration. Can feel a mis or hisitation. This past weekend I spent a couple hours pulling out all the Plugs and wires and took everything down to the NAPA store were again we went over everything. Decided to upgrade the wires to the premium set. And I installed everthing once again. Truck runs Great. No codes for the first couple hours then without fail PO300 again, Thoughts?
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Monday, July 2nd, 2012 AT 6:13 PM

9 Replies

Tiny
SATURNTECH9
  • MECHANIC
  • 30,870 POSTS
So no codes or issues before the plugs and wires you said?Its strange that changing the plugs and wires caused a random missfire not a certain cylinder missfire. Your checked to make sure you didnt close or change the gap of of the plugs?Also checked to make sure non of the plugs got cracked?
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Monday, July 2nd, 2012 AT 6:28 PM
Tiny
OLDHAMMER47
  • MEMBER
  • 5 POSTS
Yes, We checked them when I purchased them originally, then I checked them again when I removed them this past weekend and took them to NAPA for the second time to verify the gap was correct and that none of them were cracked, just to verify that I wasnt seeing things.
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Monday, July 2nd, 2012 AT 6:33 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,870 POSTS
All the wires are routed correctly?I have seen a lot of clogged injectors cause random miss fires there are even bulletins for that. But you said no issues before changing the wires and plugs. So what made you change the wires and plugs?I could see the plugs for maintance but I never change the wires till they fail.
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Monday, July 2nd, 2012 AT 7:41 PM
Tiny
OLDHAMMER47
  • MEMBER
  • 5 POSTS
Why wait for disaster, trucks running great and you want to keep it running great! The Plugs and wires had 120k miles on them which I believe is double thier life expectancy. I thought I included this statement but after readin and need to add that a flashing Service engine light comes around a constant cruse at 65mph. I am thinking of going back to the AC Platiniums.
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Monday, July 2nd, 2012 AT 8:17 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,870 POSTS
Bad wires I really wouldnt call diaster I belive in changing parts when they go bad.I got 200,000 miles out of the spark plug wires on my car before I had to change them. If your getting a flashing check engine light that means its a major missfire that can damage the cat. So if all this started from changing the wires and plugs then iam stumped if the plugs are gapped correctly not cracked the wires are good routed correctly.I cant think of what else is going on your down to those two things. Your truck came stock with iriduim plugs so going to the platnuims unless there was bad plugs I dont know how that would help. Are you sure you only have the P0300 code?Also at night have you tried spraying the spark plug wires with a water bottle looking for sparks?Also is the missing worse with the ac on?
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Monday, July 2nd, 2012 AT 8:47 PM
Tiny
OLDHAMMER47
  • MEMBER
  • 5 POSTS
Honestly, I had a service mechanic look at it, while it was throwing the Code, and he too is stumped because the Truck shows no phisical signs of missfiring. It idles Great no visable misfires. Napa guys came out and also no visable misfires. There only suggestion was to take it to GM and let them play with it on a dyno machine to hopefuly detect the issue.
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Monday, July 2nd, 2012 AT 9:00 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,870 POSTS
So you cant actually feel the missfire?Even with the check engine light flashing?
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Monday, July 2nd, 2012 AT 9:25 PM
Tiny
OLDHAMMER47
  • MEMBER
  • 5 POSTS
Correct
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Monday, July 2nd, 2012 AT 9:29 PM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,870 POSTS
That makes things even more puzzling I would try to make it set a missfire code while watching the ignition spikes on a scope. To rule out a missfire due to a ignition fault if it were my car and couldnt make it set the code with the scope on it.I would try cleaning the injectors by running it off the pressurized canister running the engine off injector cleaner. Also put some cheveron techroline the good stuff injector clean in the gas tank. Like the gm bulletins say to do for random missfires that arent caused by plugs wires etc.
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Monday, July 2nd, 2012 AT 10:12 PM

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