Hanging Revs

DRKSD

Active Member

Nov 29, 1999

112

16

38

Iowa

Jun 8, 2012

#1

• Jun 8, 2012

• #1

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.

 

usaf_branham

15 Year Member

Oct 30, 2008

927

134

84

Boston

Jun 8, 2012

#2

• Jun 8, 2012

• #2

IAC, or possibly your throttle blade is out of adjustment.

 

Night Shifter

Well-Known Member

Dec 24, 2005

1,710

27

59

Daytona

Jun 8, 2012

#3

• Jun 8, 2012

• #3

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

 

DRKSD

Active Member

Nov 29, 1999

112

16

38

Iowa

Jun 8, 2012

#4

• Jun 8, 2012

• #4

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?

 

wmburns

SN Certified Technician

Aug 14, 2009

5,892

514

204

Houston Texas

Jun 8, 2012

#5

• Jun 8, 2012

• #5

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

 

DRKSD

Active Member

Nov 29, 1999

112

16

38

Iowa

Jun 8, 2012

#6

• Jun 8, 2012

• #6

wmburns said:

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

Click to expand...

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.

 

Rejekt

Member

Aug 12, 2010

546

13

18

Raleigh, NC

Jun 8, 2012

#7

• Jun 8, 2012

• #7

Mine did something similar one night, I cleaned the IAC and throttle body and all was well.

 

Dugl

Member

May 18, 2011

83

16

19

PA.

Jun 11, 2012

#8

• Jun 11, 2012

• #8

DRKSD said:

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.

Click to expand...

 

Dugl

Member

May 18, 2011

83

16

19

PA.

Jun 11, 2012

#9

• Jun 11, 2012

• #9

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.

 

trinity_gt

10 Year Member

Jan 31, 2003

3,125

81

99

Canada

Jun 11, 2012

#10

• Jun 11, 2012

• #10

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...

 

White1987GT

Member

Jun 17, 2011

32

0

16

New Jersey

Jul 2, 2012

#11

• Jul 2, 2012

• #11

I have the same exact car. The Limited 35th anniversary, and it does the same thing.

 

R

Redneckmustang2019

New Member

Nov 13, 2019

3

0

1

Lubbock Texas

Dec 8, 2019

#12

• Dec 8, 2019

• #12

Same here. Anyone come across any new info? I've read and read for 2 days now and haven't had any head way. But my stuff's clean and all seems to be working. Just hanging on shifts