Ok im having the same idle prob as others have talk about its low (650rpm) and its a rough idle shaking the car. Ive cleaned the IAC which helped for about 2-3weeks then the idle has returned. My question is about when I was cleaning it, Ive read that to test it to see if its faulty unplug the sensor wire and if the idle doesnt change then the then its sticking and hence therefore its faulty. When I tested it, pulled the sensor wire and car died instantly, so does this mean the IAC valve is functioning properly? Im just trying to narrow down my options as the cause of the Idle. Haynes manual says EGR, IAC or MAF sensors, along with ignition system. I know its not the plugs, I recently changed them to splitfire plugs and nothing changed.

Anyone know what should happen to a running engine when you disconnect a properly functioning IAC valve?

Or if nobody minds, tell me what happens when you disconnect yours, should the engine die?
Thanks all

 

Get some carb cleaner and remove your IAC spray it down real good, let it soak and then dry it off and re-install it.

THen go get a new one if it's really messed up.........but if its really messed up it would idle a bit then just die after you totally let go of the gas...so give that a shot

It's most likely not anything to do with your ignition

 

If when you unplug the IAC and the car dies, this means the base idle is too low! ie, the TB is closed too far.

To fix it, simply adjust the idle set screw on the TB itself. Before adjusting, though, be sure to mark it with a marker so you know where it is now. Turn it about an 1/8 of a turn, and thgen try again. Keep doing it until the car almost dies, but keeps running. That is where you want the screw set.

Keep me posted and I can prolly help you with this.

 

FOrgot to say I have a 2003 GT My idle is low I around 650-700, but the speed shop I use (Dallas Mustang) did adjust my idle up a little and said "it was fine and alot of stangs will idle low." So I havnt touched the idle, It seems alot of sites I have looked at say that the engine dying is normal when the IAC is disconnected, and that a sign of a faulty IAC is if there is little to no change when you disconnect it. But I did forget to mention I disconnected it after I cleaned it, I havnt had a chance to test it now to see what happens.
I also though the PCV valve could be the instigator, as Ive read that mustangs tend to suck oil through the PCV causing it to stick. Ill check both my PCV and IAC and let you know. Do you know anything about the EECs system that is used to scavange fumes, I read in Haynes that in can cause an idle problem.

 

welll...everyone is talking about their car's idleing so low...my 96gt tends to idle between 950 and 1000 somtimes a hair higher but just about 1000 is normal when im driving it however somtimes its like it doesnt know the car is warm or somthing and it will idle at 1500 this doesnt really happen that often but does happen....and somtimes i will rev up to a decent rpm and then the rpms dip down so low that it will stall or somtimes they just pop back up...also when i bought my car i was told the cars check engine lite was on but it will run fine cuz they replaced 1 of the 02 sensors...so to get the lite off the other 3 needa be done???? so they say..also i have heard that bad o2 sensors tell your car to like send enough fuel in to run at wot...
???is if i replace the other 02 sensors will i fix my prob???

 

Its probably not your O2 sensors, you would throw another check engine light if one of those was going.

As far as the idle goes, its probably the cold weather. These cars HATE cold weather. When I start up it peaks around 1400 and i have to wait a little while to let it drop to a grand, and then my idle is around 900rpm. It will still hang a little bit when I stop, but it will still drop down to the normal idle level.

Mine didnt stall out, but it sometimes does drop fairly low in the idle range. I think its more due to my underdrive pullies than anything else.

Run some seafoam through your engine and see if that fixes things. Sometimes that can cure idle issues. Other than that, check the voltage on your TPS sensor. It needs to be no lower than .98, but no higher than 1.0.

 

that was the first thing i did..replaced the tps like a couple of weeks ago and my check engine light is still on after i had a buddy clear it off cuz the code it pulled was for the tps...so not real sure now

 

Is the TPS adjusted properly?

Is the throttle body sticking?

Find out what code its throwing. I can help you out further if I have that.

 

When the weather is cold, the idle should be a bit lower. Colder air has more oxygen.

These Mustangs should not die when the IAC is unplugged. The IAC is nothing more then a valve with a motor attached to it. The motor throttles the valve open and closed as needed and directed by the ECU. When unplugged, the IAC is closed. So, no air is going through it unless it is stuck partially open. When this happens, your idle will not be low, it will be high.

I still think your idle is set too low.

 

i had this problem with my 96 and just changed my IAC and have not had a problem since i have heard many people say they are notorius for going bad on these cars

 

Well I have found most sites to say that on later model mustangs unpluging the IAC sensor should cause the car to stall out, they say a faulty valve will cause the car to idle lower and irradicaly due to the valve sticking open. unpluging the IAC will cause the valve to close and the car should die.
I just got back from checking a few things and heres what happened. I wanted to check the vacume on the PCV valve, but to get to it I had to take the air intake off os I did, I then took out the PCV valve, and started the car, it died immediatly. So I left the PCV valve out and put the air intake back on the TB, started the car and it ran hmm? The PCV valve was ok, good vacume and no sticking. I kept the car running and put the PCV valve back in place and proceeded to the IAC sensor (everything is still in place no hoses removed and nothing else unpluged) I unpluged the IAC sensor and my car died. Ok, I was still thinking about why my car 1st died with just the air intake pulled of the TB? I restarted the car with the IAC pluged back in, and I then pulled of the IAC tube that connects the IAC to the air intake (the hose with the black box in the middle) the car died. So now im thinking about the TPS or possibly the MAF? I checked the hose on the EGR valve for good measure it still has vacum. So im kinda scratching my head, but im definatly heading out for a OHM meter.
I was going to adjust my idle but its a screw thats all threads and no head, so I left it alone, but actually the idle speed on a late model 4.6 is supposed to be nonadjustable physically and it is suppose to be adjust with a scan tool.

So would somebody pull their hoses like I have while the car is running and tell me what happens, does it die or run? So seriously please noone start arguing with me about what I said about the TB being nonadj, I know everyone does it I would just like someone to tell me if their car mimics what mine dies when the pull the vacume hose, IAC sensor, and air intake.

I would really appreciate it b/c I know its alot to ask. Thanks again everybody I really do appreciate everyones helpfull advice.

 

Havnt had a chance to get a diagnostic, plus I didnt think it would throw any codes since my engine light has not come on. Some bad sensors (like the 02) will bring the light on, so I havnt hooked a scanner up yet.

 

Have you had your car tuned? Sometimes tuners will dictate what the idle speed will be but the mechanical portion of the car will suggest otherwise.

 

the engine is bone stock, just a k&N and new plugs. Figured out, that everything I had described earlier is normal due to the position of the MAF sensor. Basicall when I pulled the air intake off air was not being sucked into the engine and hence the MAF did not detect any airflow and caused the car to die. The same goes for when I pulled the vacume line from the IAC, the sensor on the IAC detects the absense of airflow and causes the valve to close hence causing the car to die. So its almost settled, with the IAC disconnected there is not power to the valve motor so the valve should close causing the engine to die. Maybe my IAC is not bad, but is having intermittent problems? Im back at square one. I may just say f*k it and buy an IAC anyway.

hope this is helping some other ppl out there