I had an odd one in the shop today. A nice, clean 2007 XKR Coupe with 53,000 miles. The man owns two Jaguars, and has had them maintained at the dealer through their warranty periods. The XKR was running so poorly, that he had the car towed in, concerned that engine damage may result if he continued driving. We found ALL of the spark plugs, but one, were literally loose in the heads, allowing hot exhaust to escape through the loose threads, burning the plugs and the coils badly. In several cases, the plugs were so badly burned, that the metal core had actually separated from the porcelain. Several of the coils were completely split and badly burned. The "one" secured plug and good coil had been replaced by the dealer to cure a misfire problem. Seem like this particular dealer technician should have looked further! Now out of his warranty period, the oversight cost his customer quite a bit. So, I got to wondering if anyone else had seen this issue with original equipment spark plugs being insufficiently tightened at the factory? This XKR was a 06/2007 production.

 

Just had mine out (2003, 70k mile, XKR) and none were loose but they were uncomfortably difficult to unscrew. In the sort of way like when they aren't quite tight enough and the threads get cruded up.

Another interesting observation is at all but one of them showed brown around the base of the ceramic, as if they had been leaking ever so slightly.

Coils all appeared in fine condition so new plugs, a little anti seize and they went back in easy. Torqued to spec.

 

I mentioned it on another post and thought I was the only one that had this problem. I had installed the set of plugs that were loose but that time I did not use a torque wrench I have been using on ever since that day.

 

If not torqued right, plugs will come loose and I had this happen on my vette.

CC, for 70k miles your plugs have been getting hot than cold (heat syncing) and after a while, you get the odd bit of oil, this is quite normal.

 

That is unfortunate, but not wise at all either. I wouldnt have driven it home from the dealer the first time the repair was made, surely it wasn't running perfectly not to notice seven cylinders misfiring randomly and the noises associated with the plugs rattling around in the heads. A shame indeed.

 

This is normal, not a leak of combustion gas and is known as corona discharge. No problem at all normally just a byproduct of high voltage ignition passing through the spark plug.