Recently purchased a 96 XJS 4.0 with 65K miles. It had a mild rough idle so I thought I would start by changing the spark plugs since I could not find any vacuum leaks. I decided to go with the Champion RC12YC’s which appear to have resolved the idle issue.
Below is a photo of the Unipart plugs I removed. Any clues for why the ceramic insulator turned brown on a couple of these plugs where it mates with the shell?
Thanks - Kevin
Generally, that indicates a poor insulator-to-body seal: it will murder modern on-plug coil packs.
Don’t ask. 
Google “Corona Effect Spark Plug”
Here’s one result:
http://www.ngk-sparkplugs.jp/english/techinfo/qa/q17/index.html
Cheers
DD
Here’s another:
http://www.jagrepair.com/images/TSB/TSB2/XJS_XJ40/18Engine%20Management/18-55%20Spark%20Plug%20“Blow-by%20Stain”.pdf
It’s a Jaguar TSB, so it’s directly related to your concern.
Spoiler alert, no need for concern.
Interesting! I was unaware of this effect. That, too, explains the failures of some on-plug coils that I’ve experienced.
