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Is your mustang stumbling, dieing at idle or idling rough?

719516 Views 285 Replies 177 Participants Last post by  2000stangredgt
I am writing this due to the overwhelming threads started about their mustangs with these symptoms stated in this title. If you do a search on here you will find many threads started because of members stangs exhibiting these signs and almost 9/10 replies back to their problems is a faulty IAC valve or Idle air control valve. It seems this is a faulty unit from ford that can go bad as little as 20k miles or less. Mine went bad around 54k. Thankfully it is a relativitely cheap part to replace and very easy as well. If your car is showing these signs this is the first part I would replace after checking for loose vaccum lines or making sure the car has a good tune up. It has been discussed to pull the part off, shoot some carb cleaner in it and throw it back on. I did this, but it is only a band aid fix and will not last forever or really that long at all. If someone can post a pic of the IAC id appreciate it. I will try to describe it. It is a cylinder shape metal part that is attached to the plenum on the drivers side with 2 bolts. It has a vaccume line attached to it and an electrical connection. Do a search for more info, and if u have any questions feel free to ask them on this thread! Thanks

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have a problem after my blower install the car runs but its pops when i hit the gas it runs and will drive but the poping is driving me nuts any suggestions guys thanks.....its a 98 gt with a bullitt intake and a p1sc procharger 2 core inner cooler a lightning 90mm maf and a anderson powerpipe and a single blade throttle body
Get thee to a good dyno tuner.

Seriously, you need a custom tune. However, IF there are OTHER underlying problems (and with that radical a change, there could be many, I won't even try to list them all), they will have to be fixed before a meaningful tune can be completed.
2003 4.6 Stumble

No, that's not normal. Vacuum leaks can be hard to find, but you liely have one. Try reading the plugs, but its likely a cracked line or bad connection somewhere. The iac is only a factor at or very near idle. Other things to check, maf, ignition parts, fuel filter, and fuel quality.
Thanks racin366. I have been searching for the cause for so long I don't know where to lok next.
Thanks racin366. I have been searching for the cause for so long I don't know where to lok next.

Well, if you've been searching for a while and haven't found it, its either small and hard to find, you don't know how to look, or its something else. You might want to try letting the car idle and very lightly spray a bit of carb cleaner on a suspect line, if the idle jumps up you found a leak. BE VERY CAREFUL with this though, carb cleaner is very flammable, and it also isn't good for the rubber lines. Small amounts will be fine, but this is a last resort method in my opinion. Your problem, with having tried so many things already, might be a good one to start a new thread. If you do, just be thorough and list details about the car and describe the problem the best you can. The better info you provide the better solutions you'll get.
Thanks so much, I appreciate your info. One thing I did not mention was when it stumbles its hard to tell if it is a miss or transmission slip. I had the car at the dealership for free oil change and they recomended I flush the transmission. I have 65,000 miles on car and have never changed trans fluid.

Thanks so much, I appreciate your info. One thing I did not mention was when it stumbles its hard to tell if it is a miss or transmission slip. I had the car at the dealership for free oil change and they recomended I flush the transmission. I have 65,000 miles on car and have never changed trans fluid.


A trans service is a good idea then, but the dealer will charge a lot. It could be a slip, but those are usually easy to detect for an experienced driver. It still sounds like a vacuum leak or ignition problem to me.
Thanks so much...I'll try to track down the vac leak again.

so i changed the aic and its still iddeling hard and it makes me want to stall..:?: my exhaust is also trowing white smoke not aalot but more than normal and i have to constantly put water in the radiator and its a new radiator .... i think my head gasket might be craking and the coolant is leaking into the engine any ideas please help i work 7 days a week and dont have time to take my engine apart to check what it reallt is:?::?::?::?::?::?:
it might be a blown head gasket. when you change your oil check to see if you can find coolant in it. there might also be a crack in your intake manifold allowing the coolant to get into the engine that way too.
Yea i checked that my car today bearly had time and yea i have coolant in the oil but now i have to open her up and see if the head gasket is broken or the intake manifold thx i didnt think of that one or see what it is for sure but thx ill post up when i find out for sure.......
Would a bad IAC sensor keep the car from starting?
well f***k me my stang started blowing white smoke out the exhaust and i turned it off immediatly and checked the dip stick and yup coolant in the oil :nono: so either head gasket cracked or crack in the intake manifold or worse :mesad: so i think i replaced the iac for no reason :frustrated:
^^^ head gasket my friend , ****** buzz, good luck , quick question though you guys reccomned a tune with a jlt ram intake , ionly have mufflers from borla
Hopefully its just a head gasket and not a cracked head or block. Its not usual to break either, but it does happen. Unfortunately you're gonna have to pull the heads to diagnose to extent. Plan on seeing a machine shop even if its just the gasket, to make sure the heads and deck are true, and having the heads freshened at the least (valve job, seals, guides) would be a good idea. Also check all the cylinder walls and what you can see of the pistons to get an idea of how the bottom end is.
dont run it to the dealer just yet. make shur ur using a good 5w30 oil in ur motor and see if that helps
dieing while hot

my daughter has a 1997 gt . it runs fine some times then some times it will die out . it starts if it has been sitting for a little bit but if i rev the engine it will start idling rough then die out not sure where to start
Mine idles real rough for some reason :smilie
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my 99 has this problem everyonce and a while, more when it's cold...when i put the clutch in the revs will dip all the way down to 500, and it will stall...i wonder if that's just from the IAC or if it could be something worse
I have this same problem except it doesn't necessarily happen when it is cold. I have already replaced the IAC. I am thinking that maybe an EGR valve or MAF sensor could be the culprit. What do you think?
Trouble shooting IAC idle problems 1996-2004

Back to the basics:

Check the battery and alternator. Almost all auto parts stores will test for free. So there isn't a reason to cut corners here. Bottom line. Today's cars simply will NOT run right without a strong battery and charging system.

Howto perform charging system voltage drop test

Check for blown fuses. Especially fuse F2.34, F2.2 and F2.8 in the Central Junction Box (CJB 1999-2004 MY).

1999-2004 MY fuse panel schedule:

Clean the Mass Airflow Sensor (MAF) using a cleaner designed for it. The sensor is delicate and easily damaged. Never use a harsh cleaner or leaves a residue. NO brake clean. NO carb/choke cleaner.

Check for other base engine problems such as low fuel pressure, clogged fuel filter, or weak ignition or misfiring. ALWAYS resolve any ignition issues before replacing any parts.

Down to business:

Look for any vacuum leaks. Even a very small vacuum leak can cause drivability issues so don’t cut corners. Inspect all hoses (PCV, breather, evaporative purge). Look for cuts, chaffing, oil soak, dry rot or any other defects that may allow unmetered air to enter. Also inspect all intake connections to be sure they are tight and leak free. Don’t forget the oil cap, oil dip stick, and valve covers. Check the fuel pressure sensor/regulator intake vacuum reference line to be sure it's connected and leak free.

If your PCV valve is old, replace it. This is important! Remember the PCV system is basically a controlled vacuum leak. Use the correct PCV valve for the application.

Check the throttle return spring. The TB must operate freely and close completely.

There are two kinds of Idle Air Controllers (IAC). One has a black atmospheric vent. The other does not.

IAC with black vent: Usually seen on older Mustangs (2000 prior). The vent supplies bypass air to allow the motor to start. It contains a number of springs and diaphragms. The vent must internally close off when the engine is running.

Do not clean the black vent IAC. The cleaning will damage the internal diaphragms. Replace it. Autozone makes a lower cost non-Ford replacement that works well.

Non-vented IAC: these can be cleaned. Be advised, that cleaning may not fix the problem or it may only work for a short period of time.

Note, if the motor has excessive PCV blow by, this can lead to a short IAC service life.

The Mustang’s idle strategy relies on the idle air being supplied by the IAC. The throttle body butterfly valve is designed to allow only a very small (if any) air to pass. Resist the urge to adjust the TB set screw for low idle concerns. It may appear to work but when the weather gets cold, a high idle will result.

A bad IAC may cause a no start condition. Try cracking the throttle to see if the motor will start. If so, suspect bad IAC.

First functionality test of the IAC: Start and allow the motor to idle. Disconnect the IAC electrical connector. The RPM’s should drop (black vent) or the motor should die out right (non-vent).

If the idle does not drop as expected then:
• The IAC is bad or dirty.
• There is a vacuum leak downstream of the IAC.
• The evaporative purge value (VMV) is stuck open. This is allowing excess air to enter via the charcoal canister.
• The throttle body butterfly valve is allowing too much air to bypass.
• The throttle body linkage is loose or worn allowing air to enter.
• The Throttle Position Sensor (TPS) is loose, bad, or noisy (not giving good closed throttle indication).
• The electrical connection to the IAC is open or shorted. Inspect the wiring. Pay special attention on boosted/modified applications that often need to extend the IAC wiring.

If you have access to an ODB2 scanner, monitor the IAC duty cycle (percent). It should change as load on the motor changes. Turn on the AC and head lights. Verify the IAC duty cycle changes in response to changing load. Also monitor the TP-MODE PID and see if it reads "closed" when the throttle is actually closed.

If the IAC duty % is very low (< 20), suspect TB set screw allowing too much air or a vacuum leak.

Here's some information on an affordable Windows based ODB2 scanner:

The IAC handles a fair amount of air. Some engines have more vapors in the PCV system than others. Excessive blow-by can contribute to shorter life span of the IAC.

For intermittent high idle concerns, suspect a problem with the TPS and/or loose/worn throttle body linkage (aftermarket TB's). Confirm the throttle return spring is in good condition.

For the 2003/2004 Cobra that dies coasting to a stop, Ford has an updated PCM calibration to resolve this issue.

Very low idle problems could be due to a weak cylinder (not pulling it's share of the load). A cylinder power balance test can help isolate. Also look for a vacuum leak in the bypass line between the air tube and the IAC.

The TPS sensor is NOT used the same way it was during the SPEED DENSITY strategy days. The Mass Air Flow (MAF) strategy uses the TPS mainly for closed throttle and WOT processing. Remember, the PCM already knows how much air is passing through the butterfly valve because of the MAF sensor. The threads on how to "adjust" the TPS to a certain value don't do anything for the 96+ model year. Save your time.

The PCM needs to know when the throttle is closed so that the idle trim strategy can be enabled.

The PCM needs to know when the throttle is at WOT so that the WOT tables/strategy can be enabled.

The bottom line is the VALUE of TPS sensor is not as important as the stability of the value/position.

Please review the following excerpt from the Ford service CD.

Idle Speed Control Closed Throttle Determination.

One of the fundamental criteria for entering rpm control is an indication of closed throttle. Throttle mode is always calculated to the lowest learned throttle position (TP) voltage seen since engine start. This lowest learned value is called "ratch," since the software acts like a one-way ratch. The ratch value (voltage) is displayed as the TPREL PID. The ratch value is relearned after every engine start. Ratch will learn the lowest, steady TP voltage seen after the engine starts. In some cases, ratch can learn higher values of TP. The time to learn the higher values is significantly longer than the time to learn the lower values. The brakes must also be applied to learn the longer values.

All PCM functions are done using this ratch voltage, including idle speed control. The PCM goes into closed throttle mode when the TP voltage is at the ratch (TPREL PID) value. Increase in TP voltage, normally less than 0.05 volts, will put the PCM in part throttle mode. Throttle mode can be viewed by looking at the TP MODE PID. With the throttle closed, the PID must read C/T (closed throttle). Slightly corrupt values of ratch can prevent the PCM from entering closed throttle mode. An incorrect part throttle indication at idle will prevent entry into closed throttle rpm control, and could result in a high idle. Ratch can be corrupted by a throttle position sensor or circuit that "drops out" or is noisy, or by loose/worn throttle plates that close tight during a decel and spring back at a normal engine vacuum.
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