If the battery was recently disconnected or run dead the Engine Computer has to relearn "minimum throttle" before it will know when to be in control of idle speed. To meet the conditions for that relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the brake or gas pedals. Also, if minimum throttle needs to be relearned, there is usually no nice idle flare-up to 1500 rpm each time you start the engine.
If the problem continues, your mechanic can drive the vehicle with a scanner connected that has a record / playback feature. The record button is pressed when the engine stalls. Because the data travels through the scanner's memory, the recording actually starts a few seconds before the button was pressed. Later that recording can be played back to watch for sensor readings that might provide a clue.
The automatic idle speed motor will also provide a clue. It is set to one of 256 positions to set the desired idle speed. For a good-running engine step 32 is typical. When playing back the recording, if the target steps dropped real low or to 0, the computer is responding to something that is making it want a lower engine speed. If you find a higher step, the computer sees the engine speed is too low and is trying to do something about it. The next step is to figure out why the engine didn't respond.
Friday, February 11th, 2011 AT 9:27 PM