Diesel fuel is prone to waxing or gelling in cold weather; both are terms for the solidification of diesel oil into a partially crystalline state. Below the Cloud Point the fuel begins to develop solid wax particles giving it a cloudy appearance. The presence of solidified waxes thickens the oil and clogs fuel filters and injectors in engines. The crystals build up in the fuel line (especially in fuel filters) until the engine is starved of fuel, causing it to stop running.

The Cold Filter Plugging Point (CFPP) is based on a standardized test that indicates the rate at which diesel fuel will flow through a standardized filtration device in a specified length of time when cooled under certain conditions. Similarly the “Low Temperature Flow Test” (ASTM D4539^[1]) indicates the winter performance of diesel with improver additives. Note that both the CFPP and LTFT temperature is some degrees above the Pour Point temperature at which diesel fuel loses its fluid character and that pumps would stop operating.

There are a number of solutions available which allow diesel engines to continue to operate in cold weather conditions. Once the diesel motor is started it may continue to operate at temperatures below the CFPP — most engines have a spill return system, by which any excess fuel from the injector pump and injectors is returned to the fuel tank. When the engine has warmed, returning warm fuel should prevent gelling in the tank.