New Spark Plugs for XJS Rough Idle

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. :confounded:


Google “Corona Effect Spark Plug”

Here’s one result:



Here’s another:“Blow-by%20Stain”.pdf

It’s a Jaguar TSB, so it’s directly related to your concern.

Spoiler alert, no need for concern.

1 Like

Interesting! I was unaware of this effect. That, too, explains the failures of some on-plug coils that I’ve experienced.