Fuel Filter The fuel filter is located in the engine compartment on the left hand side. The fuel filter cleans fuel before the high pressure pump and prevents premature wear to delicate components. Insufficient filtration can cause damage to pump components, pressure valves and fuel injection nozzles. To prevent paraffin residue from clogging up the filter at low temperatures, there is a bimetal valve in the return line. This valve prevents heated fuel residue from mixing with cool fuel from the tank. Incorporated into the fuel filter is a fuel pressure sensor. This sensor measures the pressure of the fuel being delivered to the high pressure fuel pump. This enables the ECM to reduce the fuel injection quantity at excessively low inlet pressures to the point where engine speed and rail pressure are reduced accordingly. The volume required at the inlet of the high pressure pump is reduced. This makes it possible for the inlet pressure to the high pressure pump to rise back to the required level. At an inlet pressure of < 1.5 bar, the high pressure pump can be damaged due to inadequate fill. Another pump protection feature is that should the inlet pressure difference between the inlet and the return fuel lines ≤ 0.5 bar develops the engine can stall suddenly.