Hello all, This is my first post on here so please bare with me a bit, So I have a 98 F150 4.6v8 a month ago I had the spark plugs and wires replaced, two weeks ago it got a noticeable misfire accompanied by a backfire every so often, Brought it to the mechanic and they said they replaced the number 3 & 4 plugs and the skip went away., 5 days later... Skip is back.
Yesterday I put in 8 new motorcraft plugs, got misfires on 3,4 and 7.
Today pulled them all back out and made sure everything had the proper gap which they where not, some under .050 and some over .060, they are all at .055 now. (hood says .052-.058

All but one misfire is gone which is number 4. Any idea's what would cause this to keep happening or better yet how to fix it.

they did do a compression test on 3 and it was fine and after they swapped plugs the skip went away.

 

Did you clean the engine especially with water during this time?
They don't like getting tubbies, can cause problems.