why my mustang will idler proper on 1990 Ford Mustang GT

at first start up

Asked by for the 1990 Ford Mustang GT
The 1990 Mustang engine idles properly due to electronic fuel control. Prior to the computer controlled fuel systems, engines relied on mechanical chokes and adjustable screws to set idle speed and air fuel ratios at idle. These mechanical systems were temperamental at best and often caused improper, rough, fast, slow, or erratic idle.