Two things come to mind. First, observe if you get an "idle flare-up" when you start the engine. It should go to 1500 rpm right away, then drop to around 800 rpm after a few seconds. If it does not do this, suspect an air passage plugged with carbon or the Automatic Idle Speed Motor isn't working.
The AIS motor allows extra air to flow around the throttle blade to control idle speed. It does nothing while you have your foot on the gas pedal. When you take your foot off the gas, the computer places the motorized valve where it expects it needs to be. If the passage is partially plugged, the computer must wait to see that the idle speed is too low, then it adjusts the valve's position. Stopping slowly gives the valve time to catch up to the dropping rpm.
A similar problem can occur if the battery was recently disconnected or run dead. The engine computer looses its mind. Lost from memory are short and long-term fuel trims and minimum throttle. Without knowing minimum throttle, the computer doesn't know when to control idle speed and when to leave it up to you. It will relearn minimum throttle by taking a voltage reading from the throttle position sensor when it knows you're off the gas pedal. It knows that by knowing when you're coasting, and it knows you're coasting when it sees high manifold vacuum for seven seconds. You can get high vacuum by snapping the throttle, but not for seven seconds. The only way is to drive at highway speed, then coast for seven seconds without touching the brake or gas pedals. At that time, the computer will memorize the throttle position sensor's voltage. Any time after that when it sees the same voltage, it knows your foot is off the gas, and it must control idle speed.
Wednesday, April 15th, 2009 AT 3:08 AM