[notorious]

On D2s the quattro differential and prop shaft effectively separate fuel tank into two separate tanks (chambers) -- right and left.

Pump resides in the right chamber. Left chamber is connected with the right chamber with a pipe. Fuel is pushed through that pipe from the left chamber to the right chamber with the help of the force of unused returning fuel stream from the engine. The effect is similar to what we see in old perfume bottles with the pulveriser.

I don't know how exactly the system is implemented as I have never opened my fuel tank yet, but my hypothesis is that the 'perfume pulveriser' thing that pumps fuel from left chamber to right chamber has failed or got dirty on your car.

There must be an explanation how exactly it works (just google) and how to fix this as this should be common problem on quattro cars, not only on A8.