High idle

1999 CADILLAC DEVILLE
157,000 MILES • 4.6L • V8 • 2WD • AUTOMATIC
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MROULET1
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  • 79 POSTS
When I start my car, the RPMs are really high and if I put in gear feels like I’m breaking my transmission, lol. Sometimes it happens when I’m driving and will accelerate on its own. I will shut the car on and off to see if it will go away sometimes it works most times it doesn't. I have replaced my Throttle position sensor and have an idle air control that I haven’t put in because at this point, I don't know if I need it. please help.
Dec 6, 2021 at 5:45 AM
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SQM
  • AUTOMOTIVE REPAIR CONTRIBUTOR
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Hello,

One of the common reasons for high idle is an idle air control valve.
Overtime the spring inside the valve can go weak or become seized and allow more air flow than necessary, which leads to higher idle.
Many times, cleaning out the IAC can improve the idle quality.

So, if you have the IAC already it will be a good idea to replace it.

Take a look at this guide:

https://www.2carpros.com/articles/engine-idles-too-high

Let me know of any questions.
Thank you.
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Dec 6, 2021 at 9:21 AM
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MROULET1
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Is there any kind of relearn I have to do after installing?
Dec 7, 2021 at 7:35 AM
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SQM
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It does not call for any relearn for the IAC after replacement.
Dec 7, 2021 at 12:30 PM
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MROULET1
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Okay. well, I replaced it and it still idle high. Confused now.
Dec 9, 2021 at 4:59 AM
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SQM
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Is the check engine light on?
Dec 9, 2021 at 9:27 AM
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MROULET1
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Yes. PO440 which been on for 2 years and the newest code is PO503 Vehicle Speed Sensor A Intermittent/Erratic/High.
Dec 9, 2021 at 10:37 AM
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SQM
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Take a look at the trouble shooting guide below for the P0503. This could be related.
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Dec 9, 2021 at 2:12 PM