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Rough idle

Tiny
DORA
  • MEMBER
  • 2003 GMC ENVOY
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 140,000 MILES
My 03 gmc envoy 4.2 L starts and runs fine but when I come to a stop at a stop sign or something it vibrates and idles real rough. The rpm's gage stays pretty much the same place. But I can put it in nuetral reverse or park and it clears right up. Or if I let of the brake and let it start to roll forward it seems to ease up some on the roughness. It has about 140,000 miles on it. Any ideas what it could be. The check engine light came on one time and it was saying that the coolant temp sensor was bad. But then the light went off and hasnt came back on. Could that have anything to do with it? Also it doent do it all the time sometimes it does it worse than others, and sometimes it doesnt even do it at all? I HAVE NO IDEA WERE TO START
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Tuesday, November 30th, 2010 AT 8:59 PM

11 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,363 POSTS
Hello Dora,

This sounds like the park plugs are worn out and need replacement. Here is a guide that will help show you how to get the problem fixed,

https://www.2carpros.com/articles/how-to-change-spark-plugs

also with that many miles I would replace the MAF sensor

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

This will help the computer monitor how much air is going into the engine.

Please let me know if you need anything else to get the problem fixed.

Best, Ken
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Saturday, April 1st, 2017 AT 4:50 PM
Tiny
DENNIS2714
  • MEMBER
  • 1 POST
  • 2004 GMC ENVOY
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 101 MILES
Had battery disconnected on 2004 Envoy, 4.2L. After reconnection, vehicle idled rough and nearly stalled when a/c kicked on. Forum stated that throttle body needed to relearn after being disconnected but could also be dirty. Cleaned throttle body and reassembled. Now even worse. Check engine light and reduced power light are now on. Everything worked fine before I disconnected battery so I don't think there is actually anything bad. What do I do?
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Saturday, April 1st, 2017 AT 4:51 PM (Merged)
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Check engine light on
https://www.2carpros.com/articles/service-engine-soon-or-check-engine-light-on-or-flashing

Have the computer scanned for code/s-you have a problem within the engine management system that caused the CEL to turn on-This is your starting point of diagnosis,finding out what's going on.
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Saturday, April 1st, 2017 AT 4:51 PM (Merged)
Tiny
FTOVES2
  • MEMBER
  • 1 POST
  • 2003 GMC ENVOY
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 73,334 MILES
Engine Performance problem
2003 GMC Envoy 6 cyl Two Wheel Drive Automatic 73334 miles

today my sevice engine light came on and it started to idle rough and shaking the car. When I was driving it, it drove smoothly. What could be the problem and would it be a simple fix and what might the cost be?
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Saturday, April 1st, 2017 AT 4:51 PM (Merged)
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
If the check engine light came on, there is a trouble code stored in the computer. What you need to do is take the vehicle to a nationally recognized parts store and have them scan the computer for codes. Most will do it for free, so there is no money out of pocket and you'll know what caused the problem.

Once you find out, let me know.

Joe
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Saturday, April 1st, 2017 AT 4:51 PM (Merged)
Tiny
KSLUSHER1
  • MEMBER
  • 6 POSTS
  • 2003 GMC ENVOY
  • V8
  • 4WD
  • AUTOMATIC
  • 81,000 MILES
Engine Performance problem
2003 GMC Envoy V8 Four Wheel Drive Automatic 81000 miles

I keep getting a P0171 (system too lean) code. I changed the wires, plugs and fuel filter. It idles real rough in the morning. After it warms up and I drive 5-10 miles or so then everything runs fine. Could it be the O2 sensors? The MAF sensor is reading 3.49 on my scanner which I believe is normal.
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Saturday, April 1st, 2017 AT 4:51 PM (Merged)
Tiny
JAMES W.
  • MECHANIC
  • 2,394 POSTS
If you have to crank it quite a few times to start it cold, check the cold start injector. If this isn't working, it will run rough until it warms up.
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Saturday, April 1st, 2017 AT 4:51 PM (Merged)
Tiny
KSLUSHER1
  • MEMBER
  • 6 POSTS
I checked the codes again and I had 4. 2 lean codes for bank 1 and 2 for bank 2. It cranks and starts fine. It just runs bad after it starts.
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Saturday, April 1st, 2017 AT 4:51 PM (Merged)
Tiny
JAMES W.
  • MECHANIC
  • 2,394 POSTS
Sounds to me like the engine temp sensor is telling the computer that the engine is warmer than it is on startup. Thus, giving the motor too lean a mixture until the actual motor temp catches up with the sensor reading and then everything is ok.
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Saturday, April 1st, 2017 AT 4:51 PM (Merged)
Tiny
KSLUSHER1
  • MEMBER
  • 6 POSTS
The engine coolant temp sensor?
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Saturday, April 1st, 2017 AT 4:51 PM (Merged)
Tiny
KSLUSHER1
  • MEMBER
  • 6 POSTS
On my scanner it is showing the ECT and the IAT at about 20-30 degrees difference. Is the IAT sensor and the MAF sensor one unit on a 2003 envoy xl with a 5.3? Mitchell 1 online manual is only showing the 4.2.
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Saturday, April 1st, 2017 AT 4:51 PM (Merged)

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