1999 mustang rough idle and turns off when coming to a stop

Without knowledge of your setup id have to ask, has your car always acted this way?

Judging by the fact you stated you removed the IAC is gonna be the culprit causing the issue as its impossible to maintain idle quality without it..


The only way you could possibly remove the IAC totally is to drill a hole in the TB flap/flaps or enlarge them to allow proper airflow when the throttle flap/flaps are in closed position then manipulate the spark and fuel tables to control the spark and fuel on decelleration to keep the engine from stalling..............

Use an Idle Stop Screw → Some setups use a mechanical throttle stop to hold idle steady without an IAC. Test Idle Behavior Before Full Removal → If your engine has large cams or aggressive fueling, removing the IAC may require extensive tuning refinements.

Heres what I would do.. Id put the IAC back in and Modify the IAC Duty Cycle → Instead of removing it, some tuners reduce IAC influence by adjusting its minimum airflow settings.

I can even do it with my moates qh chip and works like a charm.......I rescaled my IAC settings to match my 351w and bumped the IAC down to 1.999% to get max flap control and avoid stalling on decel and also not get bucking on the lowspeed shifts as the IAC is needed for the time versing extra airflow needed between shifts for a certain amount of time...







Good Luck

Just had my 94 GT 5.0 cleaned at the throttle body. Was stalling as u said. Now corrected and seems to have more get up and go. Mechanic said was 60% plugged with carbon etc.

99GTSTANGG said:

Hello everyone I need help!!
I have a 1999 mustang gt procharged, and other goodies. It idles a little rough nothing to crazy, drives amazing on the freeway, but as soon as I’m coming to a stop and push in the clutch the car dies, i will catch it sometimes and i can keep it on, but more than half of the time it will shut off and it will start up fine like if nothing happened.
I also have a SCT on it and abs is on but no other lights not even Check engine

I am the second owner, I removed the IAC and it was brand new. I adjusted the TPS to .98 and full throttle was at 4.75
I checked for vacuum leaks and can’t seem to find any.
Cleaned MAF
Has a new fuel system and tank.
I don’t know what more to do

Another thing I should mention it only dies when coming to a stop and it’s at normal operating temperature

Click to expand...

I just had my 94 5.0 throttle body taken apart and cleaned. Was stalling at stop or slowly turning a corner. Now seems to have more get up and go. Mechanic said was 69% plugged with carbon etc. Now great. Check out.