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?

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    Are saying the engine doesn't idle down ... IOW: the engine speed (not vehicle speed) does not slow down when you take your foot off the accelerator pedal and put the transmission in neutral? If you have a tachometer (shows engine speed), what does it stay at or go up to when this happens? Is this happening all the time - every time or is it intermittent? Does the engine rev up during cold start up or does it stay at idle speed? Commented Dec 16, 2015 at 17:08
  • Sorry, I do not have a tachometer on my Neon. You are correct that the engine speed does not slow down when I take my foot off the accelerator. This happens every time I drive the car. On cold start, the engine make an oscillating sound for a bit, but it sounds worse than when I have been driving it for a while. I almost sounds like it is going to stall but it has not thus far. Commented Dec 21, 2015 at 21:20

2 Answers 2


This is a common problem on some Ford Focus engines and is caused by a split air hose (it could be a crank case breather hose). The split will open up while driving, then stay open for a while when the throttle is closed, which causes the same sort of symptoms I believe you are experiencing.

  • Thanks for your answer, I will begin my search for a leak and let you know how it goes. Commented Dec 21, 2015 at 21:26
  • I think your answer will fix most of my issue so I will post my update here. I replaced the internal air controller and the throttle position sensor which made engine code go away, but did not stop the symptoms I was experiencing. I then decided to clean the throttle body itself with carburetor cleaner. Upon inspection of the downstream side of the throttle body I noticed quite a bit of oil. After removing the spark plugs I also found oil on them. Commented Dec 30, 2015 at 4:43
  • I cleaned everything best I could, replaced the spark plugs, and put everything back together, only to find that the engine would rev to max RPM as soon as I turned the ignition. After tinkering with it for a while I found the source of what I would assume to be most of the problem. The underside of the air intake hose had a large tear on it. I will be replacing it tomorrow and will post tomorrow if that fixes my issue. I suppose now is the time to hunt for where the oil is leaking into the engine. Commented Dec 30, 2015 at 4:43
  • @SeñorSandia Is the oil on the plugs just on what would be outside of the engine? Is there no oil upstream of the throttle body.
    – HandyHowie
    Commented Dec 30, 2015 at 9:40
  • My idling and oscillating issue was fixed by replacing the air intake hose, thank you very much. There was oil on both sides of the spark plug I believe, and I misspoke, the oil was on the upstream side of the throttle body. Commented Dec 31, 2015 at 4:08

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.

  • Thanks Fred, I will try your suggestions tomorrow and let you know the results Commented Dec 16, 2015 at 7:15
  • BAM. Nailed it :) Commented Dec 18, 2015 at 0:17
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    Yes it should be replaced if it is stuck. Most scanners will not show that PID. Commented Dec 19, 2015 at 21:34
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    @ Sandia The IAC valve is powered by the PCM. It will not have constant power on either wire. The signal will vary rapidly. It is unlikely that the valve has failed electrically. It is fouled with intake tar deposits. Commented Dec 21, 2015 at 1:59
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    An oscilloscope is needed to evaluate the voltage pattern. It will be a fairly high frequency 12 square wave that a voltmeter will not see. Commented Dec 21, 2015 at 18:43

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