Re: 88 mustang LX 4cyl idle problem?
Clean the throttle body with an appropriate chemical and old toothbrush, get as much of the black soot off the bore and throttle blade as possible. I usually remove the IAC valve and clean it as well. I have assumed the tune up (plugs/ wires/ cap/ rotor are all good) swap in a new PCV valve, I prefer a Ford PCV valve (have had idle issues using aftermarket stuff) and finally check your ignition timing. Once these items are taken care of, all should be good. If the problem persists you will have to start looking deeper.