Hi Tech guys. I have a 1999 Pontiac Grand Prix, 3.8L V6, no turbo, 155,000 miles. When the car is in park at idle, approximatgely 1000rpm, it will surge up to 1500rpm, then down to almost a stall, then back up to 1500rpm, back dow to almost a stall, and so on. It will do this intermittently. It runs fine at street or highway speeds, but when I brake to a stop, or in park, it can start doing this rpm surge up and down, and sometimes stall. It will start up right away after a stall.
Recent engine work includes injectors and fuel rail cleaned off-engine by service station (two weeks ago), new fuel filter when injectors and rail installed; new plugs, wires and coil pack approximately 10,000 miles ago.
Could be a bad idle air motor (see if a light gas pedal stops it) tps or vacume leak. However the injectors may of been messed up by improper cleaning or the fact that they were cleaned after 155,000 miles, same goes for the fuel pressure regulator.
Also if the battery wasn't unpluged when they did all that. Unplug it for 60 and reset the pcm memory. It may of had to compensate for a now none exsistant condition