My 04 GT wagon auto has had a rough idle since I bought it. As far as I know it's completely stock, but it may have a non stock down pipe on it.
Rough feeling is intermittent ~2 sec rough, ~2 sec normal, and will waver back and fourth.
Only happens when stopped and in gear. If I back off the brake until the car is just about to move, the car will inch forward in the rough portion of the idle.
Doesn't happen every time, but does seem to happen once it's warmed up.
Took it to the mechanic who was probably just guessing but did an upper engine clean - which made no difference.
Any clues?