Engine Stalling and/or Hesitation on Acceleration due to Faulty Idle Air Bypass Valve on Ford Mustang

If you experience difficulty starting the engine when cold, stalling at idle, or hesitations during acceleration, it may be the idle air bypass valve. This valve helps the vehicle idle steady when the accelerator pedal is not pressed.

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Average mileage: 63,500 (15,000–112,000)
2 model years affected: 1994, 2003
3 people reported this problem
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problem details
At times, it hesitates and wants to stall. Other times it runs fine. Feels like it losses power and starts jerking.It's making me crazy as I have replaced plugs , plug wires, fuel filter, and went through transmission.
2003 Ford Mustang 15,000 mi,
2003 Ford Mustang GT 4.6L with hard start and poor idle (only when warm) problem. Idle Air Control (IAC) valve was the problem and is a very common cause for idling and starting issues on these cars from 1999 through 2004.
1994 Ford Mustang 112,000 mi,
i accelerate then let go of accelerater and sound like it has a tough time staying on