What are some causes for excessive fuel consumption(avg.10mpg)
1991 xjs v12.?
New plugs,wires,fuel an air filters.

 

Welcome to the forum Kp,

I've moved your question from Jaguar Forums Advice / Feedback & Suggestion Center to XJS forum (the Advice / Feedback & Suggestion Center is only for posts about the forum). Members here with the same model will be able to help.

Please follow this link New Member Area - Intro a MUST - Jaguar Forums - Jaguar Enthusiasts Forum to the New Member Area - Intro a MUST forum and post some info about yourself and your vehicle for all members to see. In return you'll get a proper welcome and some useful advice about posting to the forum.

Graham

 

I'm interested in this as well, not terrible millage, but could be much better I believe.
Does the exhaust smell so bad of raw fuel on yours you can't stand behind in? That can't be a good sign!

 

one thing i did was get rid if the cats on my 1985 xjs , i got better performance as well as better fuel mpg, also always use premiun fuel

 

Do you do a lot of short trips? The warm up cycle on the v12 requires a lot of fuel. Heating up 600lbs of metals, 8 quarts of oil and many gallons of water requires lots of energy. The initial high fuel consumption can really distort the average. I got about 8 mpg on my v12 sedan while doing mostly shorty city style running around.

 

Is that all 'city' driving, and you're not especially gentle on the throttle? If so, that's probably about right. Even with everything 100% and driven *gently* I wouldn't expect more than 12-13 mpg in all-city driving.

Steady speed highway driving should get you into the mid-to-high teens, though.

But to answer your question: a stuck enrichment switch, skewed coolant temp sensor, clogged cat convertor, broken/unplugged vacuum hose to the Marelli ECU, excessive fuel pressure, leaky pressure regulator, leaky injectors, dragging brakes......

That's off the top of my head

Cheers
DD

 

If all else is good tune, a heavy foot in town will easily get you ~10mpg.