mustang 99 idle problem (best answer picked)?

Sounds more like throttle position senor but suggest you get computer scanned for codes and reading parameters of sensors which will be more precise than guessing, Usually turns out cheaper than guessing nd changing parts, seeing there are other sensors involved in engine management. Like MAF or Map sensor and don't always turn on comp. trouble light. Also as after thought check air filter and hose to intake plenum make sure no slits. Also possible intake plenum needs cleaning. Hope something in here helps.

Some cars have a smog sensor that causes the engine to idle high for a few seconds to burn off unused fuel. I had a Nissan that would idle high if I came to a stop too fast and pushed in the clutch too soon.

looks like a gasoline administration difficulty. It would not have an oem carburetor, does it? additionally, replace the sparkplugs, that's in all threat the difficulty. many times a vehicle will run advantageous on 5 cylinders, yet gets ornery on 4. you in all threat already have one ineffective sparkplug and yet another on its way out. yet another, appropriate, is the ignition wires. If, say, a cord has been rubbed away, and is contacting some steel someplace, then vibrating far flung from the steel, which will set it off like that. yet another difficulty may well be a governor difficulty, verify the timing belt and cylinder firing mechanism. the difficulty is worse whilst the A/C is on through fact the A/C attracts greater means from the engine. That mentioned, it rather is maximum actually a difficulty with the electrical powered gadget. initiate on the sparkplugs that could desire to apply replaced besides, then the ignition wires, then the cylinder firing mechanism (pc chip). may well be a mix undesirable battery and alternator, yet that is probably to only provide out, no longer bypass on and rancid.

I would start fresh and do maintenance. Change the air filter, fuel filter, spark plugs. Is there any engine light on? To me, it sounds like a vacuum leak, but start with maintance first