For many months now my XJR with 136 000 km has been ideling rough, hestitating when I put on the gas. Tried replacing the temperature sensor as per the JTIS manual to no avail, as well as cleaning the mass air flow sensor and throttle body, checking for air leaks. Finally took it to a local tuning shop, where they found oil in the spark plug wells plus a black mark down one of the spark plugs where it had been shorting out. They replaced the valve covers which had minor cracks around the spark plug wells, but enough to leak oil, probably also air. Replaced the spark plugs, the last shop had put in spark plugs that were too short on the top resulting in the spark jumping to the spark plug. Also replaced one of the coils. Now it runs like it should, smooth and quiet with more power and better on gas. Posted this in the event someone else has the same issues.

 

Thank you for the information!

You didn't mention any so I am surprised that all that did not trigger some check engine codes somewhere along the line pointing to spark issues.

 

Suprisingly enough there were no codes.