Plug not seated to spark plug correctly? There are alot of things it could be. http://easyautodiagnostics.com/ford/3.0L-3.8L/how-to-test-the-coil-pack-1 This is a step by step how to on testing the coil pack to rule that out.
You'll test and diagnose the Ford coil pack on the car or truck with some very basic tools. You'll need:
A test light. A multimeter.
You can use a Digital Multimeter or an analog multimeter.
An HEI spark tester.
This inexpensive spark tester is a MUST have tool to be able to correctly diagnose the ignition coil pack on your Ford (or Mercury or Mazda) vehicle with the info/tests in this article
A tool that can pierce thru' the wires insulation. Such as a Wire-Piercing-Probe.
This is one of the handiest tools ever invented.
You'll need a helper to help you crank the engine while you perform the tests. A car (or truck) Shop Manual (no, not the Owner's Manual) for the specific Ford or Mercury model that you're working on. This manual is not a must for these tests, but may come in handy if you need some specific info this article doesn't provide. Previously covered here on FCO http://www.fordcontour.org/topic/13749-misfire-please-help/