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High idle

Tiny
MROULET1
  • MEMBER
  • 1999 CADILLAC DEVILLE
  • 4.6L
  • V8
  • 2WD
  • AUTOMATIC
  • 157,000 MILES
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.
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Monday, December 6th, 2021 AT 5:45 AM

7 Replies

Tiny
SQM
  • MECHANIC
  • 6,383 POSTS
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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Monday, December 6th, 2021 AT 9:21 AM
Tiny
MROULET1
  • MEMBER
  • 76 POSTS
Is there any kind of relearn I have to do after installing?
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Tuesday, December 7th, 2021 AT 7:35 AM
Tiny
SQM
  • MECHANIC
  • 6,383 POSTS
It does not call for any relearn for the IAC after replacement.
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Tuesday, December 7th, 2021 AT 12:30 PM
Tiny
MROULET1
  • MEMBER
  • 76 POSTS
Okay. Well, I replaced it and it still idle high. Confused now.
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Thursday, December 9th, 2021 AT 4:59 AM
Tiny
SQM
  • MECHANIC
  • 6,383 POSTS
Is the check engine light on?
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Thursday, December 9th, 2021 AT 9:27 AM
Tiny
MROULET1
  • MEMBER
  • 76 POSTS
Yes. PO440 which been on for 2 years and the newest code is PO503 Vehicle Speed Sensor A Intermittent/Erratic/High.
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Thursday, December 9th, 2021 AT 10:37 AM
Tiny
SQM
  • MECHANIC
  • 6,383 POSTS
Take a look at the trouble shooting guide below for the P0503. This could be related.
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Thursday, December 9th, 2021 AT 2:12 PM

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