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2002 GMC Yukon rough idle. Occasional misfire

Tiny
B4HEROES
  • MEMBER
  • 2002 GMC YUKON
  • 5.3L
  • V8
  • 2WD
  • AUTOMATIC
  • 150,000 MILES
Had bad misfire was on multiple cylinders. Replaced plugs and wires. Did it again. Replaced fuel pump and regulator. Idle was better. Then check engine light came on and misfired again badly showed a leak and misfire on cylinder 7.
Cleared codes. No more check engine light but still occasionally idles rough and occasionally misfires. Misfire can be at start up, at idle, or while driving. Misfire lasts several minutes.
Rough idle comes and goes. By rough idle, RPMs do not fluctuate, just runs rough.
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Monday, September 1st, 2014 AT 9:08 AM

7 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Misfires are hard to diagnose and can be several things. Check for a vacuum leak an dclean throttle body plate on both sides an diac hole with choke cleaner. If it starts again have a pro diagnose it as it can be any sensor causing it as well as injectors.
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Monday, September 1st, 2014 AT 9:29 AM
Tiny
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Throttle body has been cleaned.
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Monday, September 1st, 2014 AT 1:43 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Cck for a vacuum leak then get a scanner and try to see which cylinder it will give yo if it does. Then check resistnce on injector and compression as well as vacuum leak.
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Monday, September 1st, 2014 AT 3:19 PM
Tiny
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Injectors are ok. I pulled the K&N and put in a paper filter. Cleaned MAF. Cleaned TB as well as I could (on car). Its DBW so I had to have someone hold gas down so I could clean inside. Also ran Techron through the fuel system. With each little thing, it seems to run even better than before.
The kicker is that is runs great 90% of the time. I am finding it misfires on start-up, sometimes. The misfire lasts until I shut it down and wait a few minutes. Sometimes it will start up clean, others it will start up misifiring. I wait a few more minutes. Repeat until it starts clean. Every once iin a while it will misfire in heavy traffic conditions too.
It is very intermittent. The only consistent is my mileage is lower, but I am not sure if that is from the misfires or overall.
Im leaning toward a faulty MAF or MAP. I have cleaned the MAF three times- each time it improved the way it ran.
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Monday, September 15th, 2014 AT 5:05 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
K&N filters are known for screwing up mafs because to much oil I s put on them then it gets on maf and screws it up. Have pro scan for codes other than the misfire.
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Tuesday, September 16th, 2014 AT 6:02 AM
Tiny
B4HEROES
  • MEMBER
  • 4 POSTS
P0300 0307
Not giving me much else. Since I ran the full tank with Techron and recleaned TB, the misfire on start up only seems to happen after it has cooled down from driving in heavy stop and go.
I do notice a very slight jump in RPM when I am coming to a stop. It will go down to 600, jump to 800 for a second then back down settling at normal idle speed of just under 550.
IAC still dirty or could it just need to relearn proper A/F ratios from having a bad fuel pump and dirty air filter and MAF?
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Tuesday, September 16th, 2014 AT 10:43 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
The relearn does that as your are driving don't worry about that like I said let a pro handle misfires they are very hard to diagnose andhe can test things with his scanner that you have no idea what they mean.
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Tuesday, September 16th, 2014 AT 4:03 PM

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