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