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Ask GM Tech (Cam) Your Own Question
GM Tech (Cam)
GM Tech (Cam), GMC Mechanic
Category: GM
Satisfied Customers: 29494
Experience:  GM Grand Master Technician 2007. 14 years experience.
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My 2005 GM Envoy has a rough idle when in drive care not moving.

Resolved Question:

My 2005 GM Envoy has a rough idle when in drive care not moving. when placed in park, neutral or reverse, no vibration. Vibration is also worse when AC is turned on. Check engine light is NOT on. Need help. Thank you
Submitted: 7 years ago.
Category: GM
Expert:  GM Tech (Cam) replied 7 years ago.
Welcome. This sounds like the very common and very easily corrected issue on the 4.2L engines. What I suspect has happened is the throttle plates are dirty. This reduces the air flow and because this engine does not use a mass air flow sensor, it cannot adjust the fuel or throttle angle because it does not know. At idle with the AC on, there is more load on the engine and this only amplifies the issue. What you need to do is take the upper air box off on the center of the engine. It has 2 clamps, 2 bolts on the back passengers side and a hose or 2 under it when you lift. Then using a rag with carb cleaner, wipe the throttle plates clean. Push them open and clean the bore as well. Spray some around the throttle body until it is clean. Then take your battery off while you assemble this to reset the engine computer. Start it up and I suspect the issue will be gone.
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