Mercedes-Benz W203 CDI: one side of the fuel tank does not empty (transfer pump)
Mercedes-Benz W203 CDI: why does one side of the fuel tank not empty and the engine stalls?
The tank has two halves, and fuel is moved from one side to the other by an equalising or transfer pump. If the transfer does not work, one side stays full while the other runs empty. The car then stalls or air gets into the line even though there is still fuel in the tank.
Mercedes-Benz W203: what can prevent the tank equalisation or transfer?
Debris in the equalising nozzle or equalising pump, loose connections in the tank and weak return flow can prevent the transfer. Return flow can weaken, for example, because of a worn high-pressure pump or a leak on the return side. A ground fault in the pump relay is also one possible cause.
Mercedes-Benz W203: how do you locate a tank transfer fault?
Measure the fuel return flow and compare it with a working car. Also check the tank connections, the nozzle and the pumps, as well as the pump relay ground. The fault is worth locating by measuring, because replacing the pump does not always fix it.