Foxbody Idle issues

A

AmericanMuscleEnthusiast

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Jun 27, 2023

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Jun 27, 2023

#1

• Jun 27, 2023

• #1

I have a 1991 Lx 5.0 with a AOD transmission.
My Idle issues is whenever I start the car it’ll idle fine. Once I start driving and come to a stop the car will stay in an accelerating feeling. The rpm’s are suppose to drop at 800-900 idle, it’ll stay at 1200rpm at a light/stop sign. If I put the car in park the idle will hang at 1200rpm…Once I kill it and restart it’ll idle fine at 800rpm. Sometimes it won’t do this and idle / drive fine. Randomly the car will revv by itself and drive with me on the brakes. I have a newer IAC from oreillys on there now.

 

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90sickfox

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Jun 27, 2023

#2

• Jun 27, 2023

• #2

It could be several things. Should pull codes first.

 

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Mustang5L5

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Mod Dude

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#3

• Jun 27, 2023

• #3

Dump codes. Smoke check for vacuum leaks.

 

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limp

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#4

• Jun 28, 2023

• #4

Different car ( Merkur ) but I fought this for months in my Merkur.... I actually sold the car as is, and the new owner told me it he found a plugged vacuum line that was the problem..

 

Last edited: Jun 28, 2023

AeroCoupe

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#5

• Jun 28, 2023

• #5

Surging Idle Checklist - https://stangnet.com/mustang-forums/threads/help-me-create-the-surging-idle-checklist.698148/

"By the term surging idle, I mean an idle that wanders up and down more than 50 RPM, or an idle that starts low and goes high and stays there."

 

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Knuckle Dragger

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Aug 3, 2023

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oregon

Aug 8, 2023

#6

• Aug 8, 2023

• #6

AmericanMuscleEnthusiast said:

I have a 1991 Lx 5.0 with a AOD transmission.
My Idle issues is whenever I start the car it’ll idle fine. Once I start driving and come to a stop the car will stay in an accelerating feeling. The rpm’s are suppose to drop at 800-900 idle, it’ll stay at 1200rpm at a light/stop sign. If I put the car in park the idle will hang at 1200rpm…Once I kill it and restart it’ll idle fine at 800rpm. Sometimes it won’t do this and idle / drive fine. Randomly the car will revv by itself and drive with me on the brakes. I have a newer IAC from oreillys on there now.

Click to expand...

my 02 did the same stuff. replaced IAC with the same stores brand. occasionally does it still, yet way better than before. i I think its only going to be completely fixed once its swapped for a Motorcraft IAC to be honest.

 

Mhalc1

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Aug 21, 2023

#7

• Aug 21, 2023

• #7

My foxbody did the same thing, rite after I got the motor running. It turned out to be a vacuum leak. As mentioned above, I would check my vacuum hoses and IAC.

 

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Potomus Pete

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Dec 29, 2023

#8

• Dec 29, 2023

• #8

You have to do the checklist. It's a constant process until you find it. Keep checking vaccum lines. Then work on electric connections. You will be happy when its good

 

KRUISR

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Apr 16, 2015

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Dec 30, 2023

#9

• Dec 30, 2023

• #9

Agreed, do the checklist and a base idle reset (with IAC and SPOUT disconnected) to set the idle.