My 02 GT just recently started ideling a little erratic hangs around 1200 rpm then drops to normal then idles a little rough. It starts and runs great with no hesitation or or anything like that. I have cleaned the IAC with no effect if I disconnect it the car will die like it should. So my question is can the IAC still be bad since the car dies when it is unplugged?
new2mustangs
Yup...
The IAC may be behaving sluggishly in its normal operating range. Here's what it looks like inside:
![Image]()
The valve is held open by a variable duty cycle square wave created and controlled by the PCM:
![Image]()
The normal (if the throttle stop screw is properly adjusted) duty cycle is as shown above, 40% to 45%, which keeps the valve partially open to pass enough air to maintain the programmed idle speed. When added load (generator, power steering, AC, etc.) is applied to the engine the PCM increases the duty cycle of the IAC control signal to make it open further and pass more air, when such loads are removed it allow the valve to close.
There is also an electronic "dash pot" function that opens the IAC almost all the way when you decelerate with a closed throttle. It is also opened a bit when coasting to a stop, keeping the engine around 1100 rpm to insure power steering fluid pressure for the brakes.
So, short story--if it is sticking or sluggish the idle can behave as you describe. FWIW Ford states clearly in the shop manual that it "cannot be cleaned and must be replaced". If you car has more then 85k miles then there is very good possibility it is failing...
The IAC may be behaving sluggishly in its normal operating range. Here's what it looks like inside:
The valve is held open by a variable duty cycle square wave created and controlled by the PCM:
The normal (if the throttle stop screw is properly adjusted) duty cycle is as shown above, 40% to 45%, which keeps the valve partially open to pass enough air to maintain the programmed idle speed. When added load (generator, power steering, AC, etc.) is applied to the engine the PCM increases the duty cycle of the IAC control signal to make it open further and pass more air, when such loads are removed it allow the valve to close.
There is also an electronic "dash pot" function that opens the IAC almost all the way when you decelerate with a closed throttle. It is also opened a bit when coasting to a stop, keeping the engine around 1100 rpm to insure power steering fluid pressure for the brakes.
So, short story--if it is sticking or sluggish the idle can behave as you describe. FWIW Ford states clearly in the shop manual that it "cannot be cleaned and must be replaced". If you car has more then 85k miles then there is very good possibility it is failing...
cliffy - i have a new tps and iac on my gt, but at idle in drive, there seems to be the slightest misfire or 'bump' that usually results in a very small muffled 'pop' coming out of the exhaust. sometimes while coasting down to a stop, these pops cause a slight bucking. i was throwing an o2 code, and replaced both the fronts and it stopped the code, but the slight misfire is still there. anything above idle and it seems to go away. i recently installed a new throttle body, but it never popped like this until just recently. any ideas?
sorry op - to answer your question i was experiencing similar symptoms when my iac went bad. it would die if i unplugged it, but after replacing the iac - everything smoothed out very nicely. also, when i installed a new throttle body i damaged my tps and had similar issues. once again - a new tps solved that. you could try one part at a time, and if it doesnt help, im sure most stores would be willing to return it for you.
@ bullitt,
thats what i was thinking too. quite literally everything has been replaced but those due to $$$. anyone know of a proper way to see if theyre bad? i believe the ford dealer said they might be able to help. but its such a slight misfire that most people dont even notice until they drive the car, or if i point it out to them. also seems to come and go, more or less severe depending on the day.
@ bullitt,
thats what i was thinking too. quite literally everything has been replaced but those due to $$$. anyone know of a proper way to see if theyre bad? i believe the ford dealer said they might be able to help. but its such a slight misfire that most people dont even notice until they drive the car, or if i point it out to them. also seems to come and go, more or less severe depending on the day.
battery was pulled just before the bama 93 octane tune. all mods listed in my garage have been on the car for a while. i just started experiencing the slight misfire after a particularly hard session of wide open throttle - about the same time the o2 code popped up. im running bbk o2 extensions if it makes a difference - but with the code gone i figured everything must be okay. tried going back to stock tune - same thing.
Have you checked the spark plugs and COP boots? Running WOT can knock a lot of combustion chamber deposits loose and sometimes foul the plugs. I ask about the boot's because firing the plugs at WOT requires more energy than at lower loads and speeds, that can make a COP partially discharge through an older/hardened boot the to plug well--and once such a path is established it can become an alternate path of least resistance for the COP's discharge...
Your Ford dealer can do a COP stress test, but I replaced all 8 of my COPs on 02445 with Accels for $160. Wouldn't be a bad idea to do a tune up if you replace all 8.
I have no idea with this car.-
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