IAC for sure or what???

FastBird91

Registered User
My car idles perfectly until it warms up. The warmer the car the worst the idle. At about the "o" or "r" on the temp. gauge it will idle like it wants to die, about i would say once out of every 10 or so times. When it does this and i give it some gas it sticks at a higher rpm and really takes awhile to get back down to proper rpm. When i unplug the iac while the car is running the idle changes a bit but does not die. I was wondering if it means for sure the iac is bad when you unplug it while the car is running and the car dies.
Thanks!
 
Just a Guess on IAC

It seems the IAC is getting a lot of play here on the forum??? Anyway, ohm it out. Should be 7-13 ohms. If not, replace. Red probe on red wire, black probe on the other. You can take it off and clean it with carb cleaner as well, and check for non-sealing piston. There is an adjusting torx screw under a white rubber plug on the TB (at least on my '94 SC), that can be adjusted to cause a smooth decenging engine rpm after bliping the throttle. Adjusted out too far and idle hangs the drops. Adjust too far in and idle is poor.
 
check vaccum leaks.

i'd check for vaccum leaks too. looks like your bird is getting air from other place other than the aic. just my $.02
 
Hey man i had a idle problem simular to that and it was the mafs. So pull the plug on the mafs and see what it does. If it dies its not the mafs. But if it stays running it is the problem. Thats my $.02.
 
Save the money on your IAC.. If you unplugged it and the car still idled, you have a problem elsewhere. Sounds like a slight vaccuum leak somewhere or the throttle plate is haning open. It should be fully closed when its on the stops.

Actually, that's not entirely true.. check for throttle bias and vaccuum leaks first. If there is a pinhole type leak in the IAC itself, it could cause what you describe.
 
Had to replace 3 iac's on my na 3.8 stang.. 2001, man it was a good car.. but too much cash form me when i was 19... darn you insurance!!!
 
I thought the IAC acted as a Choke?
If so then it will not kill the engine once it is warmed up.
Should only kill the engine when it is cold and/or warming up.
A bad IAC will cause a high idle and usually accompanied by the infamous "Dying Goose Honk" upon shut off.
Also it will "Honk" when unplugged and running (When cold).

Sounds like a vacuum leak.. try butane unlit torch around your SC Collar nut, IC Tubes and what few vac lines you can get to without having to remove the cowling.

Here's a great How-To on boost/vac leaks.. http://www.sccoa.com/forums/showthread.php?t=19261
 
IAC a Choke?

I think not. The throttle place in the TB is completely closed at idle. The IAC is opened by the PCM a specific amount to allow intake air to bypass the closed TP. There is also an adjustment in the TB housing to "fine tune" the bypass air stream in order to achieve the best idle and deceleration characteristics. A choke on the other hand, shunts off air to a carburetor, and richen the air fuel mixture. Thereby making an engine easier to start. For fuel injection cars, during startup, the injectors inject excess fuel just like a choke would, but is controlled by the PCM. Usually, once the O2 sensors are at operating temperature, the fuel mixture is adjusted accordingly, and the richness upon startup switches to the proper air/fuel mixture. The IAC and the injectors work together to that end, and are controlled by the PCM. Further adjustments will be made to the A/F ratio as the engine warms up and reaches OT. The MAF sensor and TB work together above idle. The base voltage readings of these are used to set them both in "range" of one another, and to work best when accelerating, and WOT. Although, WOT is a separate function of the PCM, having the proper "scale" on the MAF, and the TPS allows the proper timing for this pre-programed function of WOT. Basically, a smooth transition from sensor data to pre-programed operation. Hence, hesitation in acceleration are usually due to a problem with one or both the MAF or the TPS. This of course is not accounting for ignition problems, that can also cause poor acceleration, as well as a poor idle.
 
discipled1 said:
...You can take it off and clean it with carb cleaner as well, and check for non-sealing piston. There is an adjusting torx screw under a white rubber plug on the TB (at least on my '94 SC), that can be adjusted...
The carb cleaner thing is a REAL BAD IDEA. Be careful as some cleaning agents will eat the thin & brittle plastic bellows inside the IAC, causing it to rupture and not function. If it works for you then you got lucky because I, and several others on here, have not been so fortunate and have had to buy new ones after "cleaning" them.
 
Brittle IAC Plunger

Thanks, I didn't know that. Mine was pretty tough though. Still flexable. Perhaps becoming brittle is due to age. I was really careful not to stress the fragle looking "plastic?" that seats to the bypass opening. I know that this has to be a good seal for the IAC to work properly. Carbon build up will cause leaking, and a poor idle due to air/fule mixture at idle being off. Once I was sure the piston was sealing, I adjusted the torx screw on the TB to just where the RPMS would smoothly decend from bliping the throttle back to idle. If the adjusting screw is too far out, RPM will hold at about 1000 RPMS for a bit and then return to idle. I've notice that when I engage my transmission now, I don't get the delayed compenstation in RPM, or the dipping below the 550 RPM and back up. It goes right to 550 and stays there. Also, when I barly touch the gas pedal now, the car is much more responsive.

My experience with the MAF is it affects the power curve when the sensing elements are dirty. Since I'm an old guy, I would equate the feeling to that of a dirty air filter on a carb engine.
 
then what is suggested to be used when cleaning the IAC???? what spray is harsh enough to eat away the gunk, but not damage the unit.
 
I think cleaning the iac with carb cleaner will do more harm than good. The accordian baffle will deteriorate after the carb cleaner gets on it and then you will need a new one.

Also, the iac allows air to the engine when the throttle is closed. If the iac is good, it should kill the motor when unplugged, as it won't be allowing any air flow. A bad one would allow it to keep running.

That could be the problem, but like earier stated, use an ohm meter to check it.
 
Most IAC's I am familiar with do not change position unless a signal is sent to them from the computer. If you unplugged it while the engine was running, there should be no change. At least the Mustang types are a stepper motor, power to open, power to close. If the engine starts to run poorly after you unplug it, I'd guess it's because it was compensating for something else (vacuum leak?) and now it can't.

The manual specifically says not to use carb cleaner on it. It says to scrape off any carbon, or replace it. Easy to say...
 
As David stated:
NCredSC said:
..If the iac is good, it should kill the motor when unplugged, as it won't be allowing any air flow. A bad one would allow it to keep running..
When you unplug a bad IAC on these cars, the car SHOULD die. If it doesn't, you will hear the air "Honking" through the hole, thus letting you know you have a hole in the diaphram and this will confuse your EEC giving false O2, MAF, and TPS ghost codes.
The car cannot idle correctly with this unplugged because there is no way for air to get passed the TB butterfly when it is closed, other than the IAC.
I'm sure some have cleaned their IAC's with success, but in my experience I have not had good luck cleaning them with ANY cleansers, especially Carb Cleaner. My opinion is just leave it alone unless it is already bad in which case cleaning it will not help anyways.
 
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