Hanging Revs

DRKSD

Active Member
Nov 29, 1999
112
16
38
Iowa
Before I start, I did do a search.

I've checked all the vacuum hoses, no apparent leaks. I've done the battery disconnect/headlight knob on computer reset to clear the memory.

3 weeks ago I installed a Trick Flow Track Heat upper and lower intake manifold, a Dragon 75mm throttle body, and a JLT cold air intake.

Now I have hanging revs. Revs will hang at the position the gas is let off at, then slowly drop down to 1800, hang there for a moment, drop down to 1500, hang there for a moment, drop down to 1200, and then gradually lower themselves down to a proper idle.

Anyone out there who has diagnosed or worked on this sort of problem in the past, please help to point me in the right direction here.
 
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When you disconnected the battery, was that before the install of the new parts or after you had the hanging idle? Does the car have a tune on it? Anytime i have ever disconnected the battery from my car i get the exact thing happening to my car..usually lasts a few days then goes back to normal.. basically its the computer/tune relearning everything and compensating for the new parts. Even without a tune, after disconnecting the battery the car should go through the hanging idle cycle.

If it doesnt clear up then, as stated before, check the IAC. also check and see if the TPS sensor is at the correct voltage. if its off a little it may cause hang idle/idle issues.

Hope that helps :nice:
 
I do not have a tune on the car yet. I have an sct 4bank chip in the garage but am waiting to put it in until after I've been able to install the LT headers, X pipe and Catbacks.

I disconnected the battery again a week after installation because I was not seeing a decrease in the hanging revs, in hopes of the computer relearning the Idle Trims.

If I do check the IAC/throttle blade, what am I looking for, exactly?

I've also located this page today, http://mustangforums.com/forum/4-6l...w-to-iac-rev-mod-fixing-the-hanging-rpms.html this post in particular : http://mustangforums.com/forum/7132443-post38.html .

Is there any validity to this mod/fix they are talking about?
 
It would be handy to know what the IAC duty percentage is.

When was the last time the PVC valve was replaced?

Here's some information on how to trouble shoot IAC/Idle issues.

http://www.allfordmustangs.com/foru...-dieing-idle-idling-rough-14.html#post2260649

I was literally just starting to read that site you just linked to. Found it on another forum that you'd posted on. I purchased the car 6000 miles ago, it now has 56k. I can only assume the pvc valve has not been replaced ever?

I don't think I have the capability to check the IAC duty percentage.
 
Before I start, I did do a search.

I've checked all the vacuum hoses, no apparent leaks. I've done the battery disconnect/headlight knob on computer reset to clear the memory.

3 weeks ago I installed a Trick Flow Track Heat upper and lower intake manifold, a Dragon 75mm throttle body, and a JLT cold air intake.

Now I have hanging revs. Revs will hang at the position the gas is let off at, then slowly drop down to 1800, hang there for a moment, drop down to 1500, hang there for a moment, drop down to 1200, and then gradually lower themselves down to a proper idle.

Anyone out there who has diagnosed or worked on this sort of problem in the past, please help to point me in the right direction here.
 
On our 98 GT , I had the same hanging idle issues....I know I'll get flamed fpr suggesting this but , the hose attached to the IAC is 1/2 dia, so I took a plain penny and drilled a hole in it that is s;lightly smaller than 1/4" and held it over the fitting on the IAC as I pushed the hose back on. (restricting the air flow) If you like the response of a 1969 Boss, you'll tolerate this mod.....and it's completely reversible.
 
Disconnect the IAC valve. How does the car idle?

Adjust the throttle stop screw, if necessary, to attain an idle of 500-600RPM. Re-connect the IAC and give the system some time to re-learn the closed-loop idle parameters.

BTW, don't clean throttle bodies. There is no flow benefit to it and you can remove an elastomer sealant in the TB used to seal the plate in the closed position. If you remove that you'll always be struggling with idle issues...