I have a 2001 Plymouth Neon, it has a manual transmission. Recently the car has started to not stop the engine when I shift into neutral. If I shift into neutral while moving, the engine will rev like it is moving faster (because it is not in gear and can spin freely). After I slow the car down a bit the revving sound starts to oscillate, then stops when the car stops. The car also feels like it is shaking right as I take the key out. Does anyone know what might have caused this and how to fix it?
The most common cause of this symptom is sticking of the automatic idle control valve. It is located on the throttle body right above the throttle position sensor. The idle control motor opens and closes to allow more or less air to bypass the throttle blade. The more air it lets through the higher the engine speed will be. It is controlled by the PCM. When the engine is cold it opens farther for increased speed and then when the engine is warm it lets less air in to hold the speed at 700 rpm. If a scan tool is available that will show idle speed step counts check the count when the idle speed is too high. If the number is low or zero the PCM is trying to lower the speed. In this case the idle speed motor is likely stuck and not responding to the close command.
If the count is high then the PCM is commanding the speed to be high. In the this case try this. Remove both battery cable connectors from the terminals and hold them together for about 30 seconds and then reconnect them and retest the idle. Caution this will clear all memories from the vehicle including the radio security code.
These tests will find the cause of this symptom most of the time.