Why does my 2004 mustang not idle when cold?

Answer 1

A/F mix is off. When cold has to be richer and more air to maintain ratio. Generally if too rich or lean you get a code, you may have a borderline condition that only reveals itself when it is.

Possible cause-too much air (lean) examine air intake hose carefully for any rips or holes at elbow especially and all vacumm hoses for a split. Any extra air sucked into engine and not metered by maf in your intake will lean it out.

Check fuel pressure at port. Injectors rarely fail, but your fuel pump might e going out- psi drops before you notice-causing your problem, especially if it sounds louder (another sign)

3d and least likely-your IAC (intake idle valve) a valve on your throttle body that allows extra air in behind the butterflies during a cold start. These can get gummed up with carbon and stick. Search articles on this after you've ruled out the first 2.

Fourth possibility-compression lower than threshold. Worn engine, but you should be getting misfires with this one.