High idle at first start

MROULET1

MEMBER
1999 CADILLAC DEVILLE
4.6L
V8
2WD
AUTOMATIC
157,000 MILES

Every time I start my in the morning the RPMs start at 1,000 go up and down then it will slowly start getting higher and higher almost 2,000 sometimes. Feels like if I’m pressing the gas and when I put it in a gear it slams because it’s already accelerating. Then car will then pretty much accelerate on its own. I will do all kinds of stuff like shut the car on and off sometimes that works. I’ve done changed out da IAC and the TPS and it’s still doing the same stuff. I really need someone help.

Sunday, December 12th, 2021 AT 6:53 PM

1 Reply

JACOBANDNICKOLAS

MECHANIC
110,176 POSTS

Hi,

If you have already replaced the IAC, you may simply have an engine vacuum leak. Allowing unmetered air into the engine can play havoc with the idle speed.

By chance, do you have a live data scan tool? If you do, can you let me know what the short-term fuel trims are when this is happening?

Take a look through this link. It explains the most common causes of a high idle. One thing covered is the IAC, but just bypass it and move to the next topic.

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

Let me know if that helps or if you have other questions.

Take care,

Joe

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Sunday, December 12th, 2021 AT 9:25 PM