for the first 30 seconds after you start the motor in a diesel and 10 seconds on the petrol the sub tank transfers fuel to the main tank. as above post have mentioned fuel capacities accurately. The sub tank will manually pump fuel over one you have used a 1/4 tank otherwise it faults. If there is a fault in your sub tank the orange sub tank light will stay on after ignition.
run your fuel down and try it at 3/4 full.