The injection pump’s high-pressure stage and the injection nozzle are manufactured with accuracies of several thousandths of a millimeter. As a result, contaminants in the fuel can lead to malfunctions, and inefficient filtering can cause damage to the pump components, delivery valves, and injector nozzles. This means that a fuel filter specifically aligned to the requirements of the fuel-injection system is absolutely imperative if trouble-free operation and a long service life are to be achieved. Fuel can contain water in bound form (emulsion) or unbound form (e.g., condensation due to temperature changes). If this water gets into the injection pump, corrosion damage can be the result. Distributor pumps must therefore be equipped with a fuel filter incorporating a water accumulator from which the water must be drained off at regular intervals. The increasing popularity of the diesel engine in the passenger car has led to the development of an automatic waterwarning device which indicates by means of a warning lamp when water must be drained.