In the past 6 months I have twice changed the spark plug in the #3 cylinder of my 96GL. Both times the car would show symptoms of a misfire for quite a while (2 months) before it would throw a CEL. Each time the #3 plug showed visible signs of arcing and upon replacement the car drives fine. I guess my question is what could cause this to happen? The wires are off of my SVT and only have about 50K Km on them. The first plug to fail was a six month old Autolite and the second was a used Motorcraft again off of my SVT with about 50K Km on it. When I changed the plug again this weekend I decided to use dielectric grease on the plug insulator and in the plug boot (I did this for all 6) in hopes of solving the problem. Is there a chance that the lack of dielectric grease caused the issue or is the #3 wire gone bad? I should also mention that the reason I changed the factory original wires for my used SVT wires was in an attempt to fix the misfire problem when it first started to happen earlier this year, leading me to believe that the wires are fine. Have a look at the pics of the plugs to see the arcing. Also, is the change in color (tanning) of the insulator outside the cylinder a cause for concern?
