A problem in the idle speed control system or transmission is the usual cause of this symptom. Almost all modern vehicles utilize computer controlled idle speed control. This system automatically compensates for loads placed on the engine, such as the air conditioning compressor or heavy electrical loads. A failure in the automatic transmissions torque converter clutch that prevents it from disengaging on deceleration, can also cause this symptom.
The first step in the diagnostic process is a visual inspection of the throttle body. Inspect the throttle body for binding or sticking linkage. If the vehicle is port fuel injected, a visual inspection of the throttle body for excessive carbon build up will be necessary. Remove the rubber duct that attaches to the throttle body. Inspect for black carbon build up around the throttle body. If the throttle body requires cleaning, a spray cleaner is available at any auto parts store. Spray the cleaner on a clean cloth towel. With the engine off, open the throttle and wipe the carbon from the throttle body plate and bore. If the throttle body is clean and functioning properly, the vehicle may have a defective idle speed control motor or valve. Refer to a manufacturer specific repair manual for specific test procedures.
PRECAUTIONS, TIPS, and NOTES
Use caution when working around hot or rotating engine parts. If the symptom is only present while driving then coming to a stop, the transmission torque converter should be inspected for proper operation. This might best be done by a qualified transmission shop. This should only be done after the idle speed control system is found to be functioning.