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.
Engine Stalling and/or Hesitation on Acceleration due to Faulty Idle Air Bypass Valve on Ford Mustang
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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.
i accelerate then let go of accelerater and sound like it has a tough time staying on