hey everyone i just became a new member. been reading through this forum alot but now i need help. bear with me. 03 mustang gt centennial with auto tranny. i have had the car for 6 years now and it ran great up until about a year ago. one day out of the blue i went to go to class and the car started shaking badly at idle. when i gave it gas it was stumbling pretty bad up until 2000 rpm were it would clear up. shut the car off for about an hour and everything went back to normal. 2 months later the same thing but this time after it went back to normal the idle has been different. i have a slight shake at idle that doesnt seem consistent. kinda like an intermitent misfire. overall power is good but my idle is shaky wether im in park, neutral or at a red light. also my transmission seems to buck or jerk while im doing a steady speed. so i have replaced the following....motorcraft plugs, all 8 coils and boots, cleaned IAC and MAF, new fuel filter and i always run 93 octane. also no check engine light either and when i plug it up to my reader it just says no trouble codes detected. please help me out car only has 45k.

 

Are You Tuned For 93?

 

sounds like to me you either have a cracked plug or one of your new coils went bad ,witch is very common.

 

im currently running the 91 octane tune for it off my sct tuner. the thing is the car had the tune uploaded 8 months before the idle issue and it ran great before. so i dont think its the tune. even when i remove the tune and go back to the factory tune, it still does it

 

the idle has been bad before and after the plug change and COP change. and i dont get any codes off of my reader either.

 

have you looked around on the engine to see if you may can hear a vacuum leak maybe around the plenium, or maybe its a vacuum line with a small hole in it / this will also run you ragged as well an not throw a code .

 

one more question for ya , does it have a aftermarket plenium an if so what brand ..

 

No. the only engine mod is a bbk cai that I installed way before the problem started. Other than the sct tune, I run borla stingers cat back. If I had a vacuum leak, could that cause the car to violently shake once every blue moon and not throw a cel?

 

yes, most deffently

 

an also have you changed the fuel filter since you had the car ? an i got to thinking also you may want to run a can of seafoam with your next fill up .. do that for the next couple of fill ups .. this will help ..but if you haven't changed the gas filter go ahead on change it first. motorcraft or wix filters will be the best choices for you..