So having my xjs for over 6 years. its had its mini probs and what not.
my fuel mpg has never been that great iffy at most so iv started to check things over.
I was reading "the Book" last night on the Fuel injection system and i noticed two things that my car has never done, one was overrun if the idle went to high with the throttles closed, it would cycle on/off. and shut the injectors off while cruising etc..
and with the engine off if u step on the gas you could hear the injectors fire once, never would happen.

Turns out my throttle position sensor was adjusted totally wrong so for 6 years its been like this and i finally fixed it and i sure have noticed the difference i cant wait to see my mpg now. now on too the Vac advance this week.

 

Good find.

The smallest items proper adjustment/setting is so important for what is known as the "basic settings" and so many people just skim over it.

Your fuel usage should improve significantly, along with performance.