Idle Air Control

Hey everybody, I am having a little trouble with keeping RPM’s staying at 1000 when idling. The engine will idle up to 2000 rpm sometimes and continually idle anywhere between the 2 numbers. I was told that the IAC can be carbon blocked. I pulled it off and checked for carbon, I didn’t see any. Should I still replace it or is that not the issue?

I fought that gremlin for years and finally gave up, I even had it on an 87 GT Mustang V8 back in the 90's. I never did figure it out, BUT after I installed my PiMPx I had a weird surge/stumble/AFR everywhere issue and while checking into it I switched my view to a dashboard that showed throttle % and with no input from me, it would jump from 0% to around 25% and back. I had another TPS on a spare engine and putting it on solved that. This was maddening because while troubleshooting the issue you're having, I had an analog tester on it several times checking that it went from 0 - 100 - 0 smoothly and it always did. I'd look at that.
Good luck

Hi all,  not sure if these TSB kits are posted anywhere, didn’t see them in a search. 
These kits  were used mostly on motors with non adjustable tps sensors, especially the early speed density pre mass airflow sensor fuel injection mustangs like your 87.
 Besides the sludge issues of the IABP valve , If the camshaft or heads were upgraded sometimes the idle speed was unstable and using the throttle stop screw for more air would raise the tps voltage too high.

I guess the tp sensors do seem rather delicate... I remember on my TC the engine ran no problems but the transmission 3-4 shift solenoid was erratic shifting back and forth at cruising speed and it too was just a bad tps sensor.

If the tps sensor is good and you have followed all the recommendations here for cleaning the IABP then setting up the base idle,  tps closed throttle voltage, and there is no vacuum leaks, maybe one of these could be worth a try, if it can be sourced.
 I guess they could also be made fairly easily from a old IABP base if you are crafty.

Here is the TSB 91-25 
(Sorry for the wrinkles in the paper.. it’s been folded in a box for 30 years )