I have a 03 f150 with a 4.6L. I had a PO357 code so I replace coil pack and sparkplug. It still idles rough and code still there. I used wd40 to crack out old sparkplug. Could excessive oil when replacing with new cause misfire. I checked wire harness there is power going in and out to coil pack. Also I noticed new sparkplug was slightly longer than old one. Not sure what to do.
thanks

 

What brand plug and coil did you use? Are you sure you changed the right one? #7 is the 3rd one back from the front, driver's side.

How many miles on the rest of the plugs? If more than about 100k km, I'd change them all with Motorcraft plugs.

Best coils to use are Motorcraft, Denso, or Delphi.

 

I used a NAPA brand coil pack and NGK sparkplug. I should have added that when I first had code PO357. I changed just the ignition coil and truck ran awesome for about 2days. My mechanic said sparkplug could have carbon tracking so I then changed it. It didn't seem to have a carbon tracking visual but I put new plug in anyway. Yes it was cylinder no.7 driver side 2and from back of engine.

 

You should be using Motorcraft plugs.

 

A P035x fault code meas you have an electrical circuit fault in the COP's firing circuit. That means it could be a bad connector, a faulty harness wire, or a PCM circuit fault

You have to do the proper diagnostic in order pinpoint the problem, guessing rarely works.

 

So I pulled out ignition coil that I just purchased about 2weeks ago and its cracked. So that explains misfire. Buying a Delphi brand, hopefully it lasts longer Lol

 

It’s not uncommon to get a “bad” spark plug either. These trucks are very sensitive with spark plugs and give a misfire feel. Definitely use motorcraft like mentioned above or Brisk racing plugs if you want to spend the $