rough idle

2003 GMC ENVOY
140,000 MILES • 6 CYL • 4WD • AUTOMATIC
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DORA
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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
Nov 30, 2010 at 8:59 PM
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STRAILER
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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
Apr 1, 2017 at 4:50 PM
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DENNIS2714
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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?
Apr 1, 2017 at 4:51 PM (Merged)
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RASMATAZ
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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.
Apr 1, 2017 at 4:51 PM (Merged)
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FTOVES2
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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?
Apr 1, 2017 at 4:51 PM (Merged)
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JACOBANDNICKOLAS
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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
Apr 1, 2017 at 4:51 PM (Merged)
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KSLUSHER1
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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.
Apr 1, 2017 at 4:51 PM (Merged)
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JAMES W.
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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.
Apr 1, 2017 at 4:51 PM (Merged)
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KSLUSHER1
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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.
Apr 1, 2017 at 4:51 PM (Merged)
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JAMES W.
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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.
Apr 1, 2017 at 4:51 PM (Merged)
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KSLUSHER1
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The engine coolant temp sensor?
Apr 1, 2017 at 4:51 PM (Merged)
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KSLUSHER1
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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.
Apr 1, 2017 at 4:51 PM (Merged)