Fuel filter preheating system (BMW 5 Series E39)

At low ambient temperatures, the fluidity of diesel fuel decreases due to the crystallization of the paraffin contained in it. Diesel fuel becomes thick as honey. In this regard, oil refineries add additives to diesel fuel intended for use in the winter period, which improve the fluidity of the fuel and allow the engine to start at temperatures down to -15°C (or down to -22°C when using Super Diesel fuel).

However, these additives are not always sufficient to ensure flawless engine operation, so the fuel filter is equipped with an electric heater. Thanks to this, crystallization of diesel fuel is prevented even at extremely low ambient temperatures.