hi everyone i have a 2001 f150 4x4 and between 40-45 mph my truck shutters when just ridin not gunning it none of the coil packs are bad i just replaced them but cant figure out the problem any help

 

My 97 was doing the same thing this morning....put a scanner on it and came up with code P0308, misfire on #8 cylinder.

The diagnostic routine takes you through a whole mess of possible causes including injector troubles, timing troubles, vacuum leaks...enough to scare the he*l out of you.

On a wild guess I removed and cleaned the IAC, Idle Air Controller, and the misfire seems to have gone away....permanently I hope.