Q&A

 Journeyman

You need to check the compression. If it in specifications? It might be a faulty injector?

vacuum or gaskets leaking?

You don't leave us much info here: coils (plural) and spark plug (singular)?? My first impression, based upon what you said, would be to question harness connections. These tend to break the hardened plastic connector locks, leaving poor connection to the coil. W/ engine running, push ea. connector against the coil, one at a time, to possibly find the faulty connection. If so, use a nylon wire tie to hold it on. Also, these coils are not very dependable; Ford updated their designs to solve problems; the old designs were sold off to aftermarket suppliers and often sold as "economy" line parts. And, if 1 spark plug, or all, were replaced, there may be a cracked plug. W/ engine idling, disconnect 1 coil at a time to listen for noticable change in engine sound/rpm's. If 1 does not change engine rpm's, that's a non- firing/misfiring cylinder. If plug appears OK, swap it w/ another; if still not fixed, swap the coil w/ another. The injector harnesses should also be considered for connection issues. Of course, all this is based upon another piece of missing information...that was this problem was not there before you replaced parts.