The throttle position sensor tells the engine computer which way and how fast the throttle is moving. A momentary dropout in the voltage reading can cause a stumble on acceleration but typically not a stalling problem. If the dropout occurs long enough, the computer will memorize a diagnostic fault code and turn on the "Check Engine" light.
The throttle blade normally closes completely, unlike on older cars with carburetors. There is a small hole in the blade that allows just enough air in to maintain a low idle speed. To vary idle speed, there is a valve in a passageway around the throttle blade. The computer commands the Automatic Idle Speed (AIS) motor to move to various positions and commands the injectors on for a longer period of time. The increased air and fuel raises idle speed. In addition, the computer causes an "idle flare-up" when you start the engine.
Without touching the gas pedal, the engine should go up to about 1500 rpm, then back down to around 800 rpm every time you start it. If that does not happen, suspect the passage around the throttle blade is plugged with carbon or the motor is not working. Since washing the throttle blade seems to affect the problem, I would guess the passage is not quite 100 percent plugged. The cleaning chemical is probably clearing just enough carbon to allow the minimum idle speed to be maintained.
To do a thorough cleaning, the idle speed motor must be removed. While it's out and plugged in, have someone turn on the ignition switch. You should see the pintle valve move in or out. That's proof the motor is working. This is not a normal motor with brushes; it has four coils of wire and a magnet in the middle that moves to various positions. If a coil is open or shorted, the computer will detect it and set a fault code. When removing the motor's two mounting bolts, be sure to use the proper size torx bit. The bolts have thread lock on them and are very tight. It's easy to round out the heads making removal of the bolts very difficult.
I would start by cleaning the passage although this hasn't been a common problem with the 4 cylinder engines. If that doesn't help, the dealer has a hand-held scanner, the DRB2, that allows him to run the rpm up to 2000 rpm in 200 rpm increments. This will determine if the AIS motor is working and if the computer is in control of idle speed.
Monday, June 22nd, 2009 AT 9:00 PM