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Q: why my mustang will idler proper on 1990 Ford Mustang GT

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(1) Answer
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.