Foxbody Idle issues

[AmericanMuscleEnthusiast]

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.

 

[90sickfox]

It could be several things. Should pull codes first.

 

[Mustang5L5]

Dump codes. Smoke check for vacuum leaks.

 

[limp]

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..

 

[AeroCoupe]

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."

 

[Knuckle Dragger]

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.

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]

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.

 

[Potomus Pete]

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]

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