Looking for ideas, my fuel was at 6.4 for a bit so I installed new plugs,(she would also hunt until warmed up) then I pulled the mas, and cleaned the throttle body in place, fuel went up to 12 mpg, but recently fell back to 7, I pulled the screws on the throttle body but the top did not want to come off so I didn’t mess with it, any ideas or thoughts? 1989 xj40.
At 7 mpg is the exhaust black when you are driving, I would have thought that it would be smelling really rich. Are the plugs black? Does the fuel pressure relief valve have fuel coming out of the vacuum connections? Bear in mind that short journeys can be really disastrous for fuel economy. My journey to and from work used to be a 3klm trip either way 10mpg. Long trips (500klm+) = 30mpg.
With it running that rich, maybe you have a check engine light and a fault code?
If not, maybe your fuel pressure regulator diaphragm has given out? You can usually check this by pulling the vacuum line from the FPR and seeing if gas spills out …otherwise, coolant sensor fried maybe?
My car is pretty thirsty too but even on a bad day more than double your results.
David,
I assume you are taking those readings from the trip computer? Over what distance?
My first reaction on reading your post was that unless you are being followed by a large cloud of blue smoke from the exhaust, perhaps you had switched the trip computer from imperial (distance in miles) to metric (distance in kilometers) and your fuel consumption reading was actually 7 litres per 100 kilometers - a very acceptable 33 mpUSg! 
Other than that I think the suggestions from others already are the things you should investigate - particularly the condition of each spark plug for sooty deposits - but the absence of a VCM ‘over-rich’ warning is puzzling.
Just some more info,new fpr, and coolant temp sensor, only fault codes are bulb failure and abs.and only short trips so far,less than 6 miles.steering needs an alignment also as ehen I got her both fromt tires were showing metal on the inside and lots of tread outside.
Also no over volts relay yet, ordered one last week.
That could certainly result in poor fuel consumption figures.
Yes indeed, but US 7 MPG? in litres that’s 33.6/100k = 3.36/10k = 0.33/1k 
Some time ago I was doing even shorter trips on a daily basis - just about 2-3 kilometers at a time in stop and go traffic, always from cold - (half the distance covered in open loop I suppose) and even though my mileage was pretty awful I still managed to get about 18-19litres/100k = 12-13mpg US
Of course, after a battery disconnect/mileage reset, the display will initially show very high consumption - somewhere up in the 30’s - but will slowly drop down (go up in mpg 
) as the distance traveled increases.
If you’re only going around the block though, don’t expect the display to dropdown/go up much at all.
Interesting, I am chasing down a battery drain issue at present, and have been disconecting the battery, I have the drain isolated to the drivers seat controls, now with that fuse out I will have to see if the mpg changes.
You can’t judge the fuel consumption on figures obtained on very short journeys, and also until you have the car running properly.
Lots of small things can combine to raise fuel consumption, including driving with the toe in / toe out adjustment incorrect.
If your tyres were worn to the metal on one edge it’s highly likely your’s will need adjustment.
I think you should wait until you have had the tracking checked and then take the car for a run of at least 15 miles so the engine reaches the correct temperature. Then starting afresh with the trip set to zero, measure the consumption over the 15 mile return journey. That will give you a far better idea of how its running.
I don’t think the battery drain issue is in any way at all related to the poor fuel consumption.
I seem to have isolated the battery drain to the drivers seat fuse, pulled and dropped .5 amps, now showing .14 which isn’t as bad.I have an appointment with my mechanic on Friday for an alignment, I hope nothing in there is shot and he can get her aligned. Then planning a drive to recheck the mileage.
That’s way more in line with short trip use, around town that’s what I get too. (16.8litres/100k)
So you’re all good.
That’s the current draw Larry (.14)
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Mmmm - yep but that is 140 milliamps and is still too high.
IIRC, the ‘dark current’ on an XJ40 should be around 40 to 50 milliamps at the most after five minutes with everything shut down and closed up.
Only fuses left to check are center console. I doent understand why pulling the fuse amd relay then just putting them back in dopped the amp draw?
David,
Don’t think you need to worry about the fuses in the centre console. They are not ‘live’ until the ignition is switched on so no circuits that those fuses serve can drain current with the ignition key removed.
As to your question, I can imagine one scenario (there may be others) for the sudden drop in current drain after pulling that fuse and relay. Suppose that the relay was permanently ‘made’ (e.g. if the relay contacts were temporarily sticking together when the relay was activated) and didn’t disengage when ignition-switched power was removed. Presumably something in the powered seat circuits would still be drawing current until you pulled the fuse and the relay then disengaged.
I seem to recall that a number of owners have had battery drain problems due to a faulty door lock heater circuit. Have you tried pulling the door lock heater relay - black base on passenger side ‘A’ pillar?
note to self - read what’s there not what I want to be there …