Plugs came out of a 2007 Santa Fe 2.7L. #2 was misfiring, I pulled the easy three (2,4,6) plugs and the #2 looked like this. What might cause this on the insulator that the others do not have? just from the misfire, or something wrong with this plug itself? I have replacement coils. Plugs have about 50K on them engine has 175K.