Operation of a diesel engine in winter conditions (Audi A6 C5)

As the temperature drops, the fluidity of diesel fuel decreases and it becomes paraffinized. Diesel fuel can become viscous, and its fluidity in this state can be compared to the viscosity of honey. As a result of thickening, the fuel filter becomes clogged. For this reason, diesel fuel manufacturers add additives to diesel fuel in winter, which allow maintaining the required fluidity and ensuring engine start at temperatures from -15°C to -22°C.

However, the use of additives does not always ensure flawless engine operation. To prevent the fuel filter from clogging in the cold season, it is heated by the so-called "return" - excess fuel flowing through the return hose back into the fuel tank. The fuel, passing through the high-pressure fuel pump, heats up. There is a bypass valve on the return fuel line, which, depending on the atmospheric air temperature, regulates the flow of fuel flowing back. At temperatures below +15°C, the fuel heated in the high-pressure fuel pump is fed to the fuel filter. When the temperature in the filter reaches over 31°C, the bypass valve closes and the excess fuel is sent directly to the fuel tank. The bypass valve is located on the upper end of the fuel filter.

(The article was copied from the website AUDImanual)