Diesel fuel can gel in winter Diesel contains naturally occurring wax that solidifies in cold temperatures. Normally the wax is in liquid form, and due to its importance, we definitely want it in the solution. When temperatures drop, wax crystals form and cling to one another. As temperatures continue to decrease, formation continues until it restricts the flow of fuel through fuel filters, eventually stalling the engine. Wax formation in fuel is commonly known as “gelling.” Depending on the fuel, gelling can occur at temperatures barely below 32ºF (0ºC).