Brown color on outside of plug

[SHOZ]

What causes this? Is it blowby? From a turbo motor.

 

[OneEyeJack]

It's discoloration from heat at the joint between two dissimilar materials. Nothing to worry about.

 

[Whimsey]

Don't know what causes it but I have the same situation on Motorcraft double platinum plugs I removed after 60,000 miles from my 1996 Ford Zetec 2L. They were running fine and really didn't have to be changed and not that worn. I didn't know this till I took them out. I had bought new plugs already so what the heck I changed them as 60,000 miles is the recommended mileage for changing plugs in this engine.

Whimsey

 

[6starprez]

http://www.ngk-sparkplugs.jp/english/techinfo/qa/q17/index.html

 

[Quest]

as I explained in another post a month ago: this is called corona staining (on spark plug ceramic body) and it's absolutely normal.

No impact on spark plug performance whatsoever.

Q.