Engine Odd Idle Variance

[Mr67Stang]

When I first fire up the car it will run at about 2000 rpm and then drop to 800-900 after 10-15 seconds. This is of course expected and makes me believe the idle air control valve is working. However, as I drive the car, whenever I let off the gas it holds at 2000 rpm for 5-10 seconds before dropping back down to the 800-900 idle. My first thought is vacume leak. I can't find one. Could my IAC be sticking? What else might cause this?

I miss my carburetor fed non computerized cars.

 

[jcgafford]

When I first fire up the car it will run at about 2000 rpm and then drop to 800-900 after 10-15 seconds. This is of course expected and makes me believe the idle air control valve is working. However, as I drive the car, whenever I let off the gas it holds at 2000 rpm for 5-10 seconds before dropping back down to the 800-900 idle. My first thought is vacume leak. I can't find one. Could my IAC be sticking? What else might cause this?

I miss my carburetor fed non computerized cars.

what year? if it is your 1990 is the throttle cable coming back or hanging up under the intake? i had to replace the transmission cable link on mine and when i put it all back together it would do the same thing. found out it was stuck on a wiring harness piece. tucked it all away and its all good.

 

[Mr67Stang]

what year? if it is your 1990 is the throttle cable coming back or hanging up under the intake? i had to replace the transmission cable link on mine and when i put it all back together it would do the same thing. found out it was stuck on a wiring harness piece. tucked it all away and its all good.

It's my 91 GT. It did this even when I had the automatic in it. I will take a look and see if the cable might be binding somewhere. I can warm it up and run it up and see if it's taking a while to retract back to idle mechanically. My assumption I have something sensor/CPU related is that it is ALWAYS ~2000 RPM and it is the same as when I first fire the car up cold almost every time I let off the throttle. (90%)

 

[jcgafford]

tps set correctly? i believe factory idle was 650-725. so even at "low" idle your car idles high. did the po screw with the throttle adjustment and offset the tps by doing so? everything is right in that area by the throttle cable, i bet someone tried to "mod" it and screwed the settings up. OR, you have a vacuum leak.

 

[jrichker]

See the "Surging Idle Checklist” for help with all your idle/stall problems. You can guess at the problem and throw parts at it, or you can use the checklist to help you find the problem quickly and inexpensively. The checklist is right here in the Stangnet 5.0 Tech forum and you don’t have navigate to some other unknown web site. It‘s free and doesn’t cost anything: at last count there were more than 103,000 visits and still climbing

The quick and easy way to dump the codes is in there too, and all you need to do it is a paper clip! The first two posts contain all the fixes & updates. At last count there were 24 possible causes and fixes for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.