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Jared D
Jared D, ASE Certified Technician
Category: Chevy
Satisfied Customers: 1861
Experience:  Shop Supervisor at County Fleet
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Allan here. Mom-n-law just gave us an 02 TrailBlazer w 4.2l

Customer Question

Hi, Allan here. Mom-n-law just gave us an 02 TrailBlazer w 4.2l st6 vortec. Engine idles at only about 600. and as long as ac is not on seems to be fairly smooth. But with ac on, the RPM remains the same and worse,,, as it shutters, shakes, misses, and acts like its going to die, especially when you first apply throttle. Once you're moving,it smooths out and runs fine. But when you let off at a stop, the RPM drops quickly,,, below 500, then resumes its shuttering. Shouldn't the idle RPM be automatically adjusted when ac kicks in,,, and and where would I find the bugger that's supposed to do that???
Submitted: 2 years ago.
Category: Chevy
Expert:  Jared D replied 2 years ago.

I had a trail blazer two days ago with the same problem. It turns out the low idle and shuddering was simply a dirty throttle body. The throttle body on this model is susceptible to carbon build up because of the crankcase re-circulation system. The repair manual's procedure is to remove and clean the throttle body, but we were able to clean the throttle body with come Wynn's throttle body cleaner (STP or any decent brand of throttle body spray will work). Follow the indications on the product, but basically you will open the throttle by hand with the engine off and give it a generous spray to try to knock off the carbon inside. Then you can have a helper inside the car start the engine while you spay cleaner in the throttle body. Use eye protection! That stuff is horrible when it gets in your eyes! The engine will try to stall so your helper will have to keep it running by reving the engine wen it tries to die. When you are done, drive the vehicle and get the RPMs up a little to run the carbon through and the idle should pick back up on it's own.

I don't think you should worry about doing an idle re-learn, but if you would like to try I can give you the procedure.

Expert:  Jared D replied 1 year ago.

I just wanted to check in and see if you needed any additional information. I hope that I have been of some assistance and would appreciate a positive rating if it have. Thanks!

Customer: replied 1 year ago.
Sorry Jared,
I haven't even had time to look at the vehicle, or try your suggestion. So please bare with me and be there for me when I do and possibly need further assistance.
Allan Landry
Expert:  Jared D replied 1 year ago.

Ok. Let me know if you need any help.