idle

i have replaced my iac valve in my 2001 ford mustang gt 4.6l because prior it wasnt holding a steady idle while in neutral or a stop. replaced it and started it up drove and stopped, then idle went way low and or died.. it is not holding a idle ... what would it be,, and what should i do. thank you
 
Did you double check there's no intake vacuum leaks? Especially around the IAC valve itself.

Double check that the correct IAC valve has been used. If the original was a black vented type it needs to be replaced with the same type.

Did you allow enough time for the PCM to learn new IAC Idle trim values? Or did you reset the leaning process by disconnecting the battery?

If so, remember that it does take some time for the PCM to re-lean new IAC idle trim values. Feather the throttle to allow the motor to run until the motor is up to full operating temperature. Then allow the motor to run for a full minute at idle with the AC off. Do the same with the AC on.

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/