burn spot on 3.0 valve cover

[kbeck]

I have a round burn spot on the valve cover on the left side of my engine. It's around the 1st valve of the left side (not sure what the order is). It's a 2002 with 141,000 miles on it. There is a rough idle at times but otherwise runs great. Any ideas as to what it can be?

 

[Rearanger]

Arcing spark plug wire would explain the burn and rough idle. In the dark when sprayed with water you can probably see the wire short out. Time for new wires. Probably not good to have an ignition wire resting there, maybe need an additional stand off or use piece of rubber hose taped around the wire to separate it from rubbing/shorting.