2001 Monte Carlo 3.4L V6, car starts, then immediately dies unless rpms are kept at around 1400rpms. While kept at 1400, the rpms will randomly jump to about 4000 for two to three seconds, then proceed to drop back down. After about 20 seconds of this, i can completely lift off of the throttle and it will idle at 2500rpms, and it will hold steady there. I recently replaced the Engine Coolant Bypass pipe, and it started to do this. the car hadnt been driven or started for four days prior to me replacing the pipe. I unplugged the MAF, IAT, and the Crankshaft Ventilation Tube during this replacement, and thats about it.

 

Gut instinct sounds like a possible vacuum issue. Connect everything back up, check for vac leaks and repair as needed.