The Dreaded Hanging Idle, I Am At My Wits End! Help

Shouter

Active Member
Jan 25, 2010
47
6
29
Fargo, ND
It happened so suddenly. It was friday night while cruising that I noticed my 03 Mustang GT started to hang on the idle at 1400 rpm for a long period of time, and would only go back down once I stopped and sat for awhile. First thing I though was the IAC, so I replaced that and no avail. Then I cleaned the MAF, and throttle body really well. Still, nothing. I had it hooked up to the scan tool tonight and while in the Idle Speed test mode, I can manually adjust the IAC, and it behaves as it should when I change the %. It idles really well at 50%, and also comes right back down to idle when it is kept at 50%. Once I exit that troubleshoot mode, the hanging idle comes right back, and the values of the IAC come down quite slowly from a rev along with the rpms. I am at my wits end here. Also, at idle, my TPS value is at .98 volts. I think that is right where it should be.

My mods are: Accufab Throttle body and Plenum, BBK cold air intake, full exhaust, H-Pipe, Flowmaster after the H-Pipe, and an SCT Eliminator Chip with unknown tunes on it from the previous owner.

Here is a video of it running with the IAC Value set at 50%

And here is a video of me driving. I take foot off the footfeet, push the clutch in, and coast to a stop. You notice it hangs for quite awhile, then drops down to below idle range before catching up to where it should be.\
 
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It And here is a video of me driving. I take foot off the footfeet, push the clutch in, and coast to a stop. You notice it hangs for quite awhile, then drops down to below idle range before catching up to where it should be.\
I have owned (3) Mustangs. 1996,2000,2003 and all idled the same way. IE 1100-1200 while the car was moving. Once stopped it will enter a slow idle.

The only difference I see is that your "fast" idle is just a little higher than mine.

The other odd thing I noticed is the IAC duty % doesn't change when the throttle is bumped.

Please tell us HOW the initial idle stop screw was set. You may be able to get some relief by closing it down just a little.

Here's some more information:
Troubleshoot IAC idle problems 1996-2004 | Mustang Forums at StangNet
 
The IAC didn't change duty % because I was in system test mode for the iac. I had control over what the % was at all times. I kept it at 50% to show it would idle where it should and not hang at all.

As far as the throttle body, I have no idea how it was set. I bought the car with the accufab TB and plenum installed. I did clean it recently as it was covered with grime. And coincidently the bad hanging idle happened after I did that.

Also, after the iac was replaced, and throttle body and maf were cleaned, I disconnected the battery for 15 minutes to make the ecu relearn. But apparently it didn't work.

Also, when I put a light up to the throttle body, light could be seen around most of the edges except the bottom where it was touching. And if I push and pull lightly on the bellcrank, there is some play back and forth in the throttle plate. I'm sure it always has had that, but just a note.

Another note, when I disconnect the iac, the Rpms noticeably drop, but not to where the engine seems like it might die.

Another concern is when I come to a stop, when the rpms drop down to low idle, it stupid well below normal before returning to normal low idle, and even died on me a couple times.
 
To add: From reading the above link about diagnosing the IAC, it states that an IAC without the vent should die upon disconnecting the connector. When I disconnect the IAC harness, it slows down, but does not die.
 
To add: From reading the above link about diagnosing the IAC, it states that an IAC without the vent should die upon disconnecting the connector. When I disconnect the IAC harness, it slows down, but does not die.
^^^^This confirms that too much base air is passing the throttle body. Either from a vacuum leak or the idle set screw being set too high.

You did not state HOW you adjusted and established the idle set screw's initial position. There should be a procedure in the after market instruction manual on how to perform this procedure. If this procedure wasn't done, I would start there.
 
I did not adjust the throttle body as it was set when I bought the car. But I will start there.

Also, it could be me paranoid, but when I crack the throttle a little bit to accelerate, I really notice an air noise. Now I assume it is the cold air intake doing it's job, but I guess I didn't think I would notice a sucking sound when the throttle is barely opened
 
Did you clean the throttle body with brake clean? Or did you use Throttle body cleaner? The TB's have a coating on them that will erode when you use brake clean/etc on them.

I made the same mistake once in cleaning one with incorrect cleaner and had to resort to placing a restrictor brass bushing (brass plug with hole drilled in it) in the IAC intake tubing to reduce the amount of air the came into the IAC and lower the idle. Took some trial and error but IIRC I ended up with around a 3/8 hole drilled in the plug to bypass air.
 
Well I made a quick restrictor plate. Fired it up, and no luck. Now it idles high and still hangs a tiny tiny bit. It drops revs faster, but idles at about 1k. So I took it off, ran it again with out the restrictor, and unplugged the iac, and it died, so I fired it up again, did it again, died again. Third time it hung on for dear life, but didn't die. Leads me to think I don't have a vacuum leak? I'm stumped once again
 
Did you disconnect the battery during this work? If so, it will take a little drive time for the PCM to re-learn new idle trim values. Normally the idle will improve as the PCM re-learns.

The fact that the motor now dies when the IAC is disconnected is an improvement. What else do you think was done to improve the test results?

What is the idle now when the car is moving? As a reminder it would be un-realistic to expect the moving/fast idle to drop below 1100-1200 RPM without a custom tune.
 
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I guess the hanging while moving isn't what I am concerned about. It is the hanging for awhile after I stop, then the cut down to below idle before it settles at normal idle.

Yes I disconnected the battery as well.

Since it is a maintenance item, I am going to replace the pcv as well today.
 
IF the problem is a no return to slow idle AND the fast idle is working this points to:
  • unstable TPS signal. See the resource link about how the TPS ratch function works and the role the TPS plays in determining closed throttle signal.
  • bad TPS sensor.
  • Loose or binding throttle linkage
  • weak throttle return spring.
  • vacuum leak around/near the throttle butterfly linkage
  • Since the slow idle logic depends upon the car's speed, IF there's a problem with the speedometer this could affect the IAC slow idle logic.
  • bad IAC is a possibility. But if the IAC passes the basic function testn(RPM drop when electrical disconnected), IMO it's unlikely to be an IAC issue.
Also expect the idle to be poor after a battery disconnect until the PCM has a chance to re-learn new idle trim values.

All (3) of the Mustangs that i have owned idle around 1100-1200 RPM's while moving. The idle drops to a slow idle a second or two after coming to a stop.
 
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I took my pcv valve of and tested if it seals one way like it's supposed to, and it absolutely does not. It blows almost as freely both ways. I'm under the assumption that it is supposed to seal completely one way, hence it being a check valve. So hopefully I found the culprit.