Was browsing here as a guest earlier today to pick up ideas. The best idea/advice was to get a readout of the Diagnostic Trouble Codes. Do that first!!! Thought I'd post my findings in case it would be helpful to someone else. It's a new one on me.

My engine had developed a severe misfire most noticeable around 1200 rpm in cruise. Code readout led me to cyl 4. Pulled coil and plug. Coil primary and secondary resistance checked normal. Inspected plug and checked gap. Gap was within spec at .050 and tip looked new (had replaced approx 20,000 mi ago). Only thing out of the ordinary were a couple of black streaks down the insulator which appeared to be arc burns (carbon tracks). Scrubbed off with a brass brush and reinstalled. Problem solved. Evidently moisture had gotten under the boot and allowed the HV to arc down the side of the plug creating the carbon trail which resulted in a resistive path to ground which remained after the moisture went away. Hope this helps some puzzled soul!

 

Did you use dielectric grease inside the boot tip to help prevent such occurances?
Did you inpect the boot for burns and conductive carbonizing and hardening?
Good luck.

 

Good to here you went the extra mile to check the spark plug for carbon tracing and solved your problem. I've noticed on this and other sites some people immediately blame the coil pac if a misfire occurs after a plug change. A NGK spark plug rep once told me the importance of checking and changing the COP boots if excessive mileage is on them. Besides carbonizing and hardening as Bluegrass 7 stated they also take form and may not reseal properly to the plug bore, allowing water into the bore. It is important to use dielectric grease as Bluegrass 7 stated.