My 1998 F150 w/ 4.6L with 117K mile had a bad miss in it. I finally changed all 8 plugs and wires one at a time until I found the "bad" one. It looked like #4 was the culprit as the plug was what I would call moderately oily. I used Motorcraft platinum plugs and Autolite wires.

All seemed good when I test drove it. No miss and lots of power. I drove it to work a day later and all was fine. I fired it up to go home and the miss was back but only about half as bad. By the time I got half the way home it started running worse and threw a P0303 (misfire in cyl #3).

I pulled the plug in #3 and it looks good. I swapped plug wire and still have a miss.

I tested the coil pack and saw this:

Primary Resistance Spec - .3 - 1.0 ohms is checking .9 and .9

Secondary Resistance Spec - 6.5 - 11.5K ohms is checking about 13.8 and 13.9.

Is this my issue? If so, why did it run ok for a while then bad again?

Any help is much appreciated!!!!!

 

I don't have access at home to check the numbers you posted, but I would suspect a coil pack. I did a tune up to my truck when I had a misfire. (it was time for the tune up anyway) It would run ok for about a mile or two, then start misfiring.

 

I think I traced it to a bad plug wire. I swapped the new #3 wire out for the old one and it runs like a raped ape!