Faulty fuel gauge readings or using heaps of fuel?

[teamster1975]

How long had the light been on before you topped up?
Reason I asked is they've got quite a large reserve! (And at £1.10+ / litre you're getting sod all for your money anyway )
Welcome to the club zantos!

[Tiny Tim]

The gauge on my D8 diesel is shite too... I can start the car, get a reading, kill it, wait 20 seconds, fire her up again and the reading can drop/rise by a mm or 2. Gooooo french electrics!

[steve_earwig]

My D8 always seems to loose fuel when it's parked overnight. No puddles on the floor either... Get home from a journey thinking I did well on fuel but the next morning "who's been syphoning my diesel?"

I not sure if the low fuel light is done by contacts on the D8 (I know it's not on the '9) but if I start it on a slope the light comes on for the whole journey but it's often not on the next time I start it on the flat. Maybe there's a circuit on it that holds the light on until the ignition's shut off

[mjb]

My D8 always seems to loose fuel when it's parked overnight. No puddles on the floor either... Get home from a journey thinking I did well on fuel but the next morning "who's been syphoning my diesel?"

I not sure if the low fuel light is done by contacts on the D8 (I know it's not on the '9) but if I start it on a slope the light comes on for the whole journey but it's often not on the next time I start it on the flat. Maybe there's a circuit on it that holds the light on until the ignition's shut off

Long answer: http://en.wikipedia.org/wiki/Hysteresis

Short answer: The way things like this typically work is instead of basing the light on a cutoff voltage coming from the fuel gauge sender where anything over say 1.2v is 'enough fuel' and under 1.2v is 'show the empty light', there's a 'grey' area. If you just have one cutoff voltage then what would happen if the fuel level puts you right on the limit? You're going to have the light flashing on and off all the time which is a sure-fire way of blowing a bulb and distracting the driver.

So what happens is the electronics say that if the voltage from the sender goes under 1.2v for even just a split second going over a bump then we'll turn the light on and if it goes over 1.5v then we'll turn it off. That way you'll need a large change in fuel level to turn the light off (wait until your light juuuuust comes on, then go down a VERY steep hill and it should turn off). If you've got enough fuel in the tank for the sender to send about 1.3v then you'll likely turn the light on when going over a bump, pulling away, going uphill, etc but when you turn the ignition off and on then it's still above the 1.2v threshold so it won't light until you go uphill again...


Disclaimer: the whole 1.2v thing is entirely made up but I'd imagine it would be approximately 10% fuel left based on a linear scale)