Questionmy 1996 Hyundai sonata gs (2.0 4cyl automatic) has 39,000 miles. While coasting at the rpms will drop to 1000 then come back up to 1500 rpms then briefly drop back to 1000 then come back up to 1500 and so on. The problem only occurs while not pressing the accelerator and happens regardless of whether or not the AC is on. I'm about to do a service on the car (oil change, air filter, spark plugs and wires, and so on) and was wondering what if anything I can do to address this issue. Thank you very much for you time.
AnswerIf you're having problems with the speedometer, you'll want to investigate the vehicle speed sensor as a likely cause. It's located on top of the differential portion of the transmission.
Otherwise, you'll want to check the throttle body and all related components well. Make sure the throttle cable has some slack and isn't holding the throttle open. Make sure the throttle is clean. And make sure the throttle position sensor is properly adjusted.
It's possible that the throttle position sensor is defective, but I wouldn't jump into replacing it without further evidence that it's the root of your problem.
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Added information:
When checking the throttle body, you'll also want to check the air bypass valve for proper adjustment and the idle actuator for proper operation.
