Fuel Gauge problem with 2004 X5

I have a 2004 diesel X5 and I have noticed that the fuel gauge seems to "stick" at just under half way and no longer reflects how much fuel is
in the tank. The only way I can tell how far I can drive is using the trip meter as I know I can get about 500 miles to a tank.
The car has been to the dealers twice and they have replaced the sender unit and the siphon pump but this has made no difference. They are now talking about changing the tank which will be very expensive for me.
I know that all the fuel is being used as I am getting the same number of miles after filling up as I did before the problem occurred. I say this because the dealer seems to think that one side of the tank may not be emptying thereby showing a correct fuel gauge reading. Also, the car has run out of fuel twice and both times I have filled the tank with approx. 95 litres. This is the documented tank capacity so there is no way that there is still fuel in one side of the tank.
I'm loosing patience with the dealer as they have "fixed" the problem twice now but still don't appear to know what is causing it.
Anyone any idea what might be causing this? Is it possible that it is a faulty fuel gauge and nothing to do with the tank? If so, how does a fuel gauge become faulty?
Thanks.
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Milleniumaire wrote:

your fuel gauge. I've had the same problem and had to replace the gauge. You have to remember that its a cheap mechanical movement that can be go bad with little abuse. Mine failed after only a few months of service. The onboard computer gets its signal directly from the fuel sender unit and its solid-state.
RJD
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RJD wrote:

I would agree. If it came down to a matter of replacing a fuel tank, I'd rely on a simple resettable trip odometer instead. In fact, lacking even that nice feature, I once drove an old ('62) Chrysler for several years relying only on recording the odometer mileage at each fillup and knowing about how far I could drive before needing more fuel. -- C.R. Krieger (Been there; done that)
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The trip computer and fuel gauge readings seem to correspond. The trip computer tells me that the range is 235 miles and this hasn't changed for the last 60 miles and reflects the amount of fuel that the gauge "thinks" is left.
Maybe this suggests that it is the sender unit that is at fault, but they've already replaced this!
I read somewhere that floats are used to measure the fuel levels in the tank. If this is the case, is it possible that the X5 has two floats; one for each tank, and that one of them is stuck high thereby giving the impression that it is always nearly full?
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Here's a link to the illustration of your fuel senders from RealOEM. It does show that you have two fuel gauge sender units although they both seem to be mounted in one tank. They may have replaced the wrong one or there may be a dent in your tank that prevents the sender from moving which may be why they want to replace the tank. http://www.realoem.com/bmw/showparts.do?model 71&mospidG739&btnr_0460&hg&fg

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On the third attempt and 200 later, my local dealer determined the faulty fuel gauge was due to a defective fuel sensor which I think they said was located in the fuel tank. After replacing the sensor, the fuel gauge dropped to show the correct amount of fuel in the tank.
Thankfully, this particular issue has been brought to an end and I await the next problem with my X5 with anticipation.
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