Iv had the fuel injectors cleaned and fuel filter changed.I notice it when im at a stop light, dont know if it does it in Park. Iv been told if it was the spark plugs or wires that the check engine light would turn on, and its not on. What should I check next???
It would help immensely if the year model, mileage, engine type, engine RPM at idle, runs smooth off-idle, etc.
What may cause this on a 93 GT may not be anywhere near what is causing it on a 2003.
Sorry its a 98 GT conv. Idles around 1300, runs smooth off idle. it has 140k on a v8 4.6l
It should not be idling at 1300 RPM. As to why it’s idling that fast that could be due to any number of things. Vacuum leak, Idle Air Control valve problem, ECM fault in the idle control, binding throttle cable, etc. (I’m pretty sure this model is cable operated rather than an electronic throttle body)
As a first step I would suggest getting the car scanned for codes. These can exist whether the Check Engine Light is on or not. AutoZone, Checkers, etc. will do this for you free.
The tachometer takes a pulse off of the ignition module and it’s possible that this lack of pulse could be contributing to the fast idle problem along with causing the tach to be inoperative.
Did this problem come on gradually or was it a matter of one day, bang, it’s there?
Sorry I can’t be of more help at this point but there are too many unknowns. The 4.6 is a very good engine and I’m making the assumption (right or wrong) that the engine is fine mechanically.
Yes the engine is great. Its a problem that gradually came about. You have been a great help. I will take it and have it scanned for codes.
OK4450 you are a genius. I checked the idle control valve and I guess it was loose. It idles about 900 right now. I took it out and got up on it, warmed it up and at the end it still idled at the same spot and no more roughness. I cant thank you enough.
Random guess and glad it’s worked out. Having an engine idle that fast is not good on an automatic transmission and brakes. The 900 still sounds a shade high to me but then again, I don’t know what spec is called for on that particular year model Mustang.
For what it’s worth and in regards to those engines, a performance shop that specializes is building them has stated they’ve seen those motors come in with 400-500k miles on them and the block was still within factory specs.
My prior Lincoln Mark and my current one both have the 4.6 (DOHC version) and they’re the sweetest running engines I’ve ever owned. My prior Lincoln had 250k miles on it when some guy ran into me and up to the point of impact it still ran like new and used no oil between oil changes.
I kept the wreck and if the right Mustang with a blown engine presents itself… 
I will call Ford in the morning to find out what it should be idling at.but it did go from 13oo to 8-900. You saved me a lot of cash and hassle bud.