Egghead Tip - Idle issues and testing the Idle Air Controller (IAC) on Fox Body Mustangs

High idle or sticking idle can be due to a bad IAC, either the plunger sticks or allows too much air in closed position.

[@eastcoastmustang1713]

Made a gasket out of cork made small holes and it worked runs amazing almost brought it to a mechanic thank you

[@willz1988]

Thanks for posting man, sadly didnt get more views but very helpful, hopefully fixes my surging idle

[@SouthernOregonReps]

So I have a stock air filter set up. When I rev my rpms, my check engine come on and off. Could this be the problem?

[@faustogonzalez8397]

TPS SENSOR is required and a Critical Input to the Computer. TPS sensor detects the Throttle P,ate angle position, therefore the Computer then responds with the correct amount of fuel via the fuel injectors. TPS is monitoring your foot on the accelerator pedal, Critical input and is required to be set at approximately 0.9 Vdc and Base Idle setting MUST be correct. Thanks for your video.

[@70sport37]

Should be able to block off holes with your fingers and drop the idle hopefully . My 86 started idleing at 2k rpm other day and it was surging before as well and dyeing when cold

[@ponyup90]

Where was the “test” part? Other than blowing in it

[@hernrafael]

great video ! nice explanation good camera views and explanation

[@chriscolvin2640]

Very helpful thank you

[@happygilmore8768]

Heres the deal...The IAC isnt calibrated as a unit...The calibration is done in the tune and what youre doing by advancing and changing things is tricking the ECU and messing with engine load , spark advance/retard and also fuel delivery timing of the fuel injectors..... Look up what ITHBMA does and why 10* of timing and a TPS setting of .098 unless changed in the ECU physically is important because of a lil thing called a checksum number........... Its very true that all IAC's leak alot of extra air when fully closed but whats funny is the IAC never fully closes however as its not suposed to see anything lower than 30% duty cycle at any given time....If it does that means the timing is advanced and the t-stop screw has been altered.. Thse plates are a temp fix as when you go to the ISC transfer settings theres a finetune function where you have to do a procedure to dial in the plate to get proper airflow........ If youre the type like me that wants the IAC to operate as intended with better accuracy heres what you do.. Pop the crimped on metal endcap off the IAC by using some finesse and also remove the electric solenoid... Unscrew the nylon plug with o-ring seal from the spot the metal cap covered... Pull out pintle and take a bevel bit tool and create a small bevel where the pintle plug rests then use lapping compound to lap them totally sealed shut.. On the end of the pintle shaft that goes up into the IAC solenoid you need to clipoff a very small amount to compensate for the bevelling and lapping of the pintle plug or it will always be forced open by the electric IAC solenoid.. Pull out pintle and clean both pieces then take the spring and stretch it out an inch longer than stock and put it all back together making sure the nylon seal and o-ring bottom out all the way and the o-ring on the IAC solenoid is there before reassemblng that.. When you plug one hole and give it a suction test youll find absolutely the most finite of a leak to zero..... The next things you need to do is get an MSD timing advance/retardcontroller for the EEC computer and install it but put the distributor back to 10* btdc... Aslong as the pip signal is pulsing at the right time as per the distributor altering the timng via the knob doesnt mess with ECU settings but advances/retards timing up to a full 15* or a 7.5/7.5 split.. If you have an A9L computer and a 5speed stick 5.0 engine you must also take note to see if you have 22kohms on pin #4 coming from the ECU plug going to the IDM circuit on the TFI module pin.....If you have an auomatic with A9P the resistor is inside the TFI module and is CCD controlled but the resistance for a 5 speed is very important as it quiets down the noise by limiting the power it gets through the IDM circuitry........

[@dntlss]

What is the other story with idle hanging on these cars? i see 5 thousand threads on this but no one seems to have the answer,i have a 91 and a 89 and have been involved with these cars since the late 80s , my 91 and 89 never did it but now my 91 has been doing it for a couple of years,I'm very mechanically inclined and haven't been able to solve it, sometimes it hangs at 2200 rpm and it takes a bit for it to come down, extremely annoying. I have a AOD on my 91 and when you come to stop because the idle hangs it feels like when you are landing on a jumbo jet and they turn the reversers on, vroooommm,lol, like the tranny is not downshifting but its not that, tranny is freshly rebuilt, the only thing not new is my IAC and i found a guy with several original new ones and i bought them all, im hanging onto them like gold, should have them by the end of this week,hoping this might help my issue. Engine is new,dizzy,TPS,radiator,throttle body,computer has been rebuild, tranny has been rebuilt,mass air is new, entire engine has been smoked, no vaccum leaks external or internal,10 pin connectors cleaned,both intake sensors new,fuel pressure regulator new,speed sensor is not new but its working fine and it has been tested,all bases covered,lol,only thing not new like i i said is the IAC so we will see at the end of the week what happens. On a forum once i read this really good post by a guy and one of the things that stood out was when he said and i quote him to the best of my recollection "some cars have this problem and some dont, sometimes it goes away on its own" he mentioned a parameter on the computer called "return to idle" or something to that matter and thats what happens sometimes but in my case the computer has been freshly rebuilt and i have tested it with another one i have, same issue,with all that said the car runs great with the exception of the vroooom at the stoplight and a little idle hunting once it comes to a stop. BTW, great post on fixing the IAC,the more detailed the better,thank you.

[@lanny222]

My 89mustang is surging pretty bad.... cat figure it out blah....

[@GreenToFeelBlue]

Ever figure it out?

[@lanny222]

Not yet sadly no... did a base idle reset and replaced my vacuum lights along with my fuel filter. I’m running out of options lol...

[@GreenToFeelBlue]

Vacuum lights?

[@jimg81]

@@GreenToFeelBlue 😂😂😂