Ok. Had plugs and wires changed about 2 months ago. NGK plugs and Aurora wires. Went about 2 months and it developed a misfire. Threw a code that indicated #7 cylinder misfire. In resetting the codes and waiting 2 days to go buy a new plug to try it as a first step replace problem got better. Misfire went away. It returned but only after the truck has run for 10 min or so. will not miss bad enough to throw a code now. I have towed a trailer and as long as i was in the 2200 to 2500 rpm range no issues. higher or lower and a stumble or misfire started .... but not enough to throw a code. Does not miss or stumble when cold ie at start up. only after about 10 or 15 min of running. have traced the plug wire and can find no damage. I would think a bad plug would indicate warm or cold so i am looking at coil pack. Wondered if anyone else has this or has seen it. going to pull the #7 plug and check for damage

 

the plug is likely fine. just replace the coil that's responsible for firing it.

 

did you use dielectric grease to water proof them? if not I'd do that. Put it on the inside of the boot real good.

 

early 4.6's have coil packs.. 2 coils with wires.

 

I used dielectric grease yes. Would a coil pack go bad on one stud only or would it fail across the 4.

 

I've never had coil packs but generally I've seen they fail pretty much on all 4. I do know those engines like motorcaft wires could also be the plug they are picky although NGK's are usually ok might try swapping the injector over to another cylinder and see if the miss moves with it, A compression test might be a in order if all else fails. look at it run at night see if there is arcing on the wire..

 

ah, gotchya

 

just check if the spark plugs are tight enough my spark plug #5 was loose, and I had P0301 and P0306

 

I am having the same problem with my 97 4.6. In the past couple years I have had about 3 misfire codes and all have been the plug. I recently got p0307 (#7) so decided to put in new Motorcraft plugs (they were autolite double plat). After a short drive I got P0307 again. I now intermittently get P0307, and sometimes just runs poorly and don't get the code. I have pulled the plug and it is not fouled. I have read that a lean mixture can cause misfires particularly at low rpm, which can be caused by the MAF sensor. I cleaned mine and did seem to get better, but is now back. I think I am going to get new wires and swap coils and see if the misfire cylinder number changes. I am then going to look into getting a OBD II code scanner to view sensor signals while driving. All I have now is a code reader to get the code and turn off the check light.