When the problem is intermittent it can be pretty hard to diagnose without a scanner. I use the Chrysler DRB3 to show the idle "steps" the idle speed motor has been placed at by the Engine Computer. There are 256 steps corresponding to totally closed to completely open for the air passage around the throttle blade. For a properly running engine step 32 is typical for maintaining the idle speed. A single cylinder misfire will result in the computer bumping it up to around step 45 to 50 to keep the speed up. The step number will give you an idea of how the system is working. If it stays on step "0", either the computer hasn't relearned minimum throttle, (which doesn't sound like the case here), or there's a vacuum leak causing the idle speed to be too high and the computer is closing off the air passage in an attempt to get the speed back down to where it should be. At the same time it shortens the injectors' pulse times to inject less fuel. A vacuum leak will result in more air but the less fuel from the computer's attempt to lower speed results in a very lean condition which is conducive to stalling.
An intermittent misfire can also cause stalling from the idle speed being too low and the computer can't respond fast enough.
The scanner will also allow you to increase engine speed up to 2000 rpm in 200 rpm increments to verify the idle speed motor is working, the computer has control of it, and the air passage isn't plugged with carbon. Even when that works, you may still want to remove the motor and clean any carbon out of the passage. The computer is programmed to operate that valve according to the known size of the passage and if carbon buildup changes the size, it isn't going to respond as expected.
You can also watch the idle speed steps on the scanner when you slow down to a stop. At slow speeds, those steps should start to increase from "0" when your foot is off the gas pedal.
Friday, August 31st, 2012 AT 7:38 PM