Anyone know anything about petrol sensors in cars

I can't seem to find any free car forums to ask my question in so I am posting it here. My car keeps telling me that I have half a tank of petrol (that I do have) then as I drive it it rapidly decreases to 0. When I mean rapidly I drove a mile and it went down 70 miles. Overnight it goes back up to half a tank of petrol does anyone know what this could be?

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  • daveyjp
    daveyjp Posts: 12,406
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    70 miles for half a tank may be totally normal for a Rolls Royce V12 being used as a limo in Westminster.
  • facade
    facade Posts: 6,910
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    If you post the make model & year of the car someone may be able to suggest something.

    In The Real Olden Days cars used a manometer on the dashboard and an air pipe from the tank, and measured the pressure caused by the height of fuel in the tank. I'm guessing it isn't one of those!
    I want to go back to The Olden Days, when every single thing that I can think of was better.....

    (except air quality and Medical Science ;))
  • I can't seem to find any free car forums to ask my question in so I am posting it here. My car keeps telling me that I have half a tank of petrol (that I do have) then as I drive it it rapidly decreases to 0. When I mean rapidly I drove a mile and it went down 70 miles. Overnight it goes back up to half a tank of petrol does anyone know what this could be?
    We had a 1975 Hillman Imp that did this.  Turned out to be a resistor or regulator in the dash I think.
  • droopsnoot
    droopsnoot Posts: 1,735
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    Do you mean the fuel level gauge, or the fuel range display? On the one hand you say it showed you have half a tank which sounds like a gauge reading, but then you say it went down 70 miles which sounds like a range reading.

    If you mean the fuel range, that's a prediction based on average MPG - if you then start the car from cold and jump into stop-start traffic for that mile, it will re-calculate the range based on it running a bit richer because it's cold, not getting optimum MPG in traffic, and so on. 
  • My car had a spate of saying I had 0 fuel then the next day going back to quarter or third etc. talking to other owners it’s a known fault where the sender unit in the tank sticks.

    I resorted to thumping the rear seat just above the fuel tank which cured it temporarily. 

    I was quoted £250 + labour to replace. I then discovered if I used decent diesel rather than the cheapest supermarket the problem went away.
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