Diesel fuel is drawn from the fuel tank through the filter by the injection pump.
The injection pump pressurizes, meters, distributes and times the fuel to the injectors. Excess fuel from the pump and injectors returns to the tank via a tank return line. Diesel fuel cools and lubricates the injection pump and injectors.
Diesel fuel circulation also aids in warming the fuel in the tank and lines thus helping to prevent waxing in cold weather.