The xjs began loosing power after having sat for 1 year in New York winter conditions. I started the car one day and it ran fine and I drove it around the corner. The next day I started the car and it ran terribly. Missing and stalling. The next day it would not start. I took the car to a Jaguar specialist who spent days checking every thing and dead ended at the temperature sensor for mixture control which would not function correctly as a function of the engine temp.
We tried a different approach and cleaned and replaced injectors. The engine ran well and the mixture was correct until I took the car 70 miles, shut it down for a week and headed back. She ran fine on the highway until I hit some BTB traffic and the engine began to loose power.
I was able to limp home at which time I checked to see if the cat converter was red hot from dumping fuel but it was cold.
At several stages of diagnosis before cleaning the injectors we checked the plugs and they were heavily carbrorised.