I bought an 05 V6 Mustang in May. The car runs fine except for one small problem...the RPM's have a mind of their own. It is fine when I'm in gear, however, once I let it idle at a stoplight or stopsign the RPM's go crazy. The go up and down anywhere from 700 to 2500, just back and forth. I have taken it to 2 different dealerships and no one can find anything wrong with it. I also had a miserable experience by calling the Ford nationwide hotline. They were rude and had no ideas for possible solutions. I was hoping someone out there had some.

 

This is a pretty new car, I'd take it back to the dealer where you bought it from, again and again[:@]. I know its a pain but they will get tired of see you, and you have a lagit reason. Think some state have a lemon law, after so many time with same problem they can't or wont fix, then I think you get your cash back or another car (or something).
Now as to the cause and I'm not sure of this now cause its a newer car then I have info for, but cars are just cars. On like lets say '99 Mustang, its a distributorless ignation system, my guess is that this one is the same, maybe a little diff but not by much. It works by a Crank Postion sensor give info to a Ignation Control module which cause the coil packs to spark the plugs. The ICM also sends a signal to the TACH and PCM. The PCM causes the spark to be advanced or delayed depending on condistions. Now my guess is that most would suspect that the ICM is bad, could be. But the way to test this is with a scaner. You see the PCM can give you the engine speed, and if it is nice and steady, then the tach gauge most be the problem. I bet the tech at these dealer are unwill to give that I try,,,I COULD BE WRONG.
Now armed with this little bit of info, go back and ask them to till you if the PCM and tach read the same? GOOD LUCK

 

YEP . just keep taking it back, An I hope you got the name of the person you spoke with on the hotline , An file a compliant, Its ppl like you that . you keep them in a job , Thats why your called a customer ,, now go get'em, Also you call the BBB, Consumer Protection Agency,NHTSA, ,, Remember its the squeaky wheel that gets the grease , good luck, An I agree with "cctxfish", theres gotta be a problem with the puther

 

No, I guessed guage, guage or ICM, but I wouldn't rule out PCM.