http://www.tomco-inc.com/Tech_Tips/ttt21.pdf#search='Idle%20Air%20Control%20Value%20information'
Everything you ever needed to know. I work at a parts store and they go bad ALL the time. I probably have 10 customers a day coming in complaining cuz they have replaced every part on their car and yet it still wont idle right. For mustangs you can get them fairly cheap though.