Read latest forums, Start and idle good motor smooth and quiet, changed plugs and coil packs, check engine code misfire #4! Runs good until load then runs rough and lag in acceleration!!! Reset Codes same problem changed coil and plug from different cylinder, that are new, no change??

 

Did you have the misfire before changing plugs and coil packs? What brand of plugs and coil packs did you use? Any other codes? An injector can cause a misfire too. And be aware that the 2005s got a batch of bad injectors. I'd put a couple of bottles of tehcron or gumout with PEA in the gas and run it through.

 

Some misfire earlier on other plugs so changed coils and plugs! Was fine then started this after a little while, ran seafoam thru it too and reset still does it!! Run Accel coils and MGK plus with no problems for at least 20,000 miles!!!

 

Use Motorcraft DG511 coils and SP546 Motorcraft plugs.

Do not use Seafoam, it's too hard on the cats.

 

Did those Accel coils come with new 'boots & springs'? If not - make SURE they get replaced, with a little dab a' dielectric grease. Boots become dry / brittle from extreme heat cycles and crack and moisture condenses in cracks and provides arc-over paths that can't even be seen.