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Surging/hanging idle on 2006 Mustang GT...IAC or TPS??

57742 Views 15 Replies 9 Participants Last post by  robheimsoth
I've scoured the forums and google and can't seem to get a straight answer. i know I'm not the only one online who has experienced idle surge problems on a 2006 Mustang GT? Somebody please tell me where the IAC is located? Is it part of the OEM throttle body? My idle seems to hang sporadically anywhere from 1,000 RPM to 1,200 RPM when I coast in neutral. It drops back down to approximately 800 RPM when I come to a complete stop. From what I gather, this is part of the "dashpot" function that is associated with the idle air control valve (IAC)?? Or is it something that could be happening as a result of a bad throttle position sensor (TPS)?

I have a manual auto with a FRPP 90mm CAI and DiabloSport tune. I tried using the stock airbox and re-flashing back to the stock tune, but didn't seem to resolve the idle surge. I also tried cleaning the MAF with MAF cleaner, cleaning the throttle body with teflon-safe cleaner and replacing the original air filter. None of the that made much difference. Anybody have any idea?? I don't wanna go throught the dealership fix this and break that, routine. I can't afford it.

This is straight from the Chilton's online manual for the 2006 Mustang GT...

The IAC valve assembly is not adjustable and cannot be cleaned, also some IAC valves are normally open and others are
normally closed. Some IAC valves require engine vacuum to operate.
The IAC valve assembly controls the engine idle speed and provides a dashpot function. The IAC valve assembly meters intake air around the throttle plate through a bypass within the IAC valve assembly and throttle body. The PCM determines the desired idle speed or bypass air and signals the IAC valve assembly through a specified duty cycle. The IAC valve responds by positioning the IAC valve to control the amount of bypassed air. The PCM monitors engine RPM, and increases or decreases the IAC duty cycle in order to achieve the desired RPM. The PCM uses the IAC valve assembly to control:

- No touch start
- Cold engine fast idle for rapid warm-up
- Idle (corrects for engine load)
- Stumble or stalling on deceleration (provides a dashpot function)
- Over–temperature idle boost
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How long does it hang up?
I didn't think the 2005-2009 GT's have IAT valves. The throttle-by-wire system regulates the Idle. :headscratch:
Correct... These cars do not use idle air control. It's entirely electronic throttle control.
Follow Up #1 - Throttle body?

So I haven't really been able to determine a pattern here. The hanging idle seems to occur randomly and sometimes it will hang briefly and other times it will hang longer before settling down. Usually, that means 1,000 RPM - 1,200 RPM while moving. Occasionally, it will slowly settle below 1,000 while coming to a stop. Completely stopped it seems to idle just fine, approximately around 800 RPM, based on the tachometer.

So I went to a dealership today and had them take a look at it. Basically, they recommended replacing the entire throttle body to the tune of $600+. With barely over 55k miles on the odometer, I find that option very disappointing. Does anybody else think their diagnosis is correct? Do I really need to replace this thing already?

Anybody have any suggestions on throttle bodies? I can buy a used OEM throttle body from a junkyard for $130 and install it myself or takeoff for around $200 from an online parts dealer. I am considering upgrading to a BBK or FRPP (161-M-9926-3V) but I'm wondering if either of those will work as a bolt-on replacement under the following conditions:

2006 Mustang GT (w/3.73 gears, FRPP axle-back mufflers)

1. Stock airbox and tune (adjusted only for 3.73 gears and 18" tires)
2. 90mm FRPP CAI, 91oct (90mm CAI DiabloSport Predator tune)
3. 90mm FRPP CAI, 91oct (FRPP Pro-Cal II tune, same as Shelby GT)

Do either of those "aftermarket" throttle bodies mentioned above require custom tuning other thant what I have listed? Thank you in advance for any information you can provide.
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Does it surge or just hang?

I have a manual auto with a FRPP 90mm CAI and DiabloSport tune.
Which is it?

Not that it matters a hole lot but typically while the vehicle is in motion the idle will hang about 1100-1200 rpm until you come to complete stop. Is this something new or did you just now notice it?
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idle hangs

sorry for the terminology. occasionally it will surge (rise and fall), but the issue i'm concerned about is hanging (idling high even when my foot is off the throttle). it first did it after my brother-in-law, a service tech, did a throttle body cleaning. i can't recall what he sprayed on it so i'll have to double-check and see if he can remember. that being said, it immediately idled high around 2,000RPM+ in neutral with my foot OFF the gas following that cleaning. to correct it he said to disconnect the battery and leave it overnight. i did and all was good in the morning. several miles later, i noticed the tendency to idle high again while moving, though only a maximum around 1,200RPMs or so. it's not real bad, but it is noticeable and i don't think it is normal.
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That can be taken care of with a good tune.
I had a problem JUST like this. What i did was clean the MAF sensor with MAF cleaner. Can buy a can at Adv. Auto parts for less than $10 bucks, lasts forever.

That helped the idle a little bit, but then it kept doing what it was doing.

So, i replaced the fuel filter. Helped a little bit, but still, had the problem.

O2 sensors were fine, so was compression and vacuum. PCV valve had some oil on it, but nothing major. Cleaned out all the vacuum lines with the MAF cleaner. (Make sure it dries for a few minutes. It evaps. faster than 99% alchohol).

After all that, it still surged. So, i cleaned the Fuel rail pressure sensor with the MAF cleaner. *Make sure you depressurize the fuel lines!* It is located on the right fuel rail. (Sorry, no pics).

After a few weeks of troubleshooting, it was still surging! But, as soon as i re flashed the computer, it worked! It idled smooth, had more power, better fuel economy. It was perfect!

I don't know what the individual problem was, but i somehow fixed it. My guess was the fuel rail pressure sensor got clogged.

Well, best of luck! Post your results. It would be exciting to know if this works for you! :bigthumbsup

*Edit* I reflashed the computer sever times during this, but only after i cleaned most of the things listed it finally worked. (My memory is a bit fuzzy on the specifics).

4000 miles later and its still running GREAT! Even installed a CAI with premium tune. No problems whatsoever.
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well, thank you for your response and enthusiasm. unfortunately, i don't know how to do half of that. i'd probably mess something up in the process. sounds like fuel rail pressure sensor might be an area to check, but no way i'll attempt it. thanks.
The fuel pressure sensor is a simple job and costs about $125. You can tell if you need one if you pull the vacuum line and smell gasoline.
Has the throttle body been cleaned. I mean cleaned with a proper cleaner and wiped down with a clean lint free rag. I would check this first by taking the air inlet tube off the throttle body, turning the ignition on (do not start car) and have someone push the gas peddle to the floor. When the throttle opens have a look with a flashlight just inside the throttle blades where they would mate against the throttle bore to see if there is a build up of grime. If it is hard to see, rub your finger in this area and see how dirty it is. I had about 30K on my car when I started getting a unstable idle and it turned out just cleaning the throttle body really good cured the problem for me.
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surging idle

I'm having the same problem with my 06' GT. It started surging to almost 3000rpm and lagging down to 400rpm. When I took it to my mechanic they said they just cleaned the throttle body, it ran fine after that until now, that was 5000 miles ago. So now at 40k miles it's doing it again only rougher this time. BTW, the car is completely stock except for an eibach lowering kit. Hopefully it's just the throttle body again. But, I can't figure out why a little grime would make it surge like that.
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My car's been dropping down to 450rpm periodically at stops, then back to 750, during warmup higher as expected. I mentioned it to my tuner (before it wasn't so bad, wish I didn't mention it..), think he changed some idle trims (maybe went the wrong way with an adjustment, wish I had gone thru all mech checks first.) and it appears worse now.. Either way, I am hard on my ride and will go clean/check everything mentioned on this thread and provide feedback. I am slightly modified so might be something different. Thanks guys for the tips
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surging hanging idle

Come to find out I had a bad throttle position sensor (tps), now it runs like a beast again.
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