GM Repair Problems? Ask GM Repair Expert Now.
Hi there, I am Cam. This is a common issue on the 4.2L engine. What happens is the system was likely reset because of the battery being remove. Then when it was hooked back up, the adaptations it had learned specifically for the throttle have been erased. It then idles differently when a load is put on such as the AC or rear window defogger. To correct this, take the upper intake plenum off, the black box that says 4.2L Vortec. There are 2 10mm bolts and 2 hose clamps. Then clean the throttle plates with a rag and carb cleaner. After that, take the battery cable off and reset the vehicle. This should fix this.