I had added this to an existing thread on the Zetec forum, but think it should probably be here instead (especially since it's a 2.3 Duratec)! Sorry to have cross posted.
My engine was running rough upon cold starts. My battery subsequently dies, was replaced and the problem went away. It then returned after three or four days.
The car is an 04 2.3l ZTW with manual transmission. It has done about 75k miles.
Since mid-October I've been experiencing this rough engine noise when starting from cold. It seems to go away after the car has been driven for about 20 minutes.
It appears while accelerating, particularly in the lower gears as revs increase. It's not constant through the revs though, so the engine will sound rough then a fraction of a second sounding normal then rough again. it does usually appear shortly before the shift gear light appears on the dashboard.
I'd describe the noise as sounding almost like a motorbike engine. The car goes from quiet and smooth to loud, rough and noisy.
It is particularly prevalent when making a right hand turn - i.e. a sharp 90 degree turn while accelerating away. For example if I turn right on red onto a fast road.
The car went in to the shop who replaced the idler kit and serpentine belt. That's achieved nothing other than lowering by bank balance by several hundred dollars.
Would anyone have any other suggestions? Why would the noise go away for days after a new battery is installed then return? Disconnecting the battery for an afternoon then reconnecting did not make it go away a second time. There's no check engine light on.
I really know little about cars, and fear spending hundreds more as the shop takes stabs in the dark while not getting this resolved.