Trip computer accuracy. Or rather, inaccuracy...

[GlennH]

Folks,

Now I'm getting a few miles in on the XJR-S I have noticed the fuel usage figure (and therefore the mpg figure) on the trip computer is out by about 20%. At the most recent fill up, the computer said I'd used 13 Gallons, but I'd actually used 15.6 Gallons. (I always brim the tank on every fill up). I don't think I dropped 2.6 Gallons on the road anywhere so I am guessing the trip figure is wrong. The distance measure is pretty close (only 1 mile out after 250 miles).
I'd be interested to know how accurate the computer is on other XJ-Ss.
Any idea what the cause of the problem might be?

Happy New Year,

Glenn.

[poprivet]

The distance information comes from the same source as the mileometer so it will be almost identical. The fueling information comes from calculating the injector pulses rather then physically measuring the fuel so variations in injector flow and pressure will affect the accuracy.
Pop

[nigel1]

Mine is pretty accurate, say within 0.1 of a gallon, and mpg matches what I calculate and what the trip comp says.
As Pop says, the trip computer works out the consumption based on injector pulses. The computer knows the injector opening time, and the number of times the injectors are pulsed. What it dosnt know for sure is the fuel pressure, it makes an assumption that pressure is what it should be.
For a 4 litre XJS, fuel pressure with engine running and vac hose connected to the regulator should be 34 to 38PSI.
Would be best to get that checked.

Happy New Year
Nigel

[GlennH]

Thanks guys, it's good to know the figure should be reliable and where it comes from.
Judging by their shiny newness, the previous owner changed both pressure regulators recently. I wonder if there is a connection?
I think the first thing I'll check are the vacuum hoses, I guess if they're leaking, the pressure will be higher than it should be.
Regards,
Glenn.

[nigel1]

Glenn,
not 100% on the V12 fuel regulators, guess you have 2 of them. mine has a small vacumm hose to the regulator, with that removed, the pressure goes from 36 to 44 PSI