Diesel fuel solidifies, or gels, at temperatures around 15 degrees Fahrenheit (-9 degrees Celsius). The gel point temperature is higher for biodiesel.
The gel point of no. 2 diesel fuel, the fossil fuel most commonly used in diesel-engine cars and trucks, typically starts around -9 or -10 degrees Celsius (15.8 or 14 degrees Fahrenheit). Biodiesel users who have converted their no. 2 diesel engines into more eco-friendly models may find that their fuel gels at temperatures as high as 20 degrees Celsius (68 degrees Fahrenheit), particularly with biodiesel fuels rendered from greases and animal fats. This gelled or crystallized fuel can cause diesel engines to seize, meaning drivers should be careful before revving their engines in colder weather.