56 Fuel System Fuel Pour Point Fuel pour point should be at least 10 F (6 C) lower than the lowest ambient temperature at which the engine must start and operate. Lower pour points of diesel fuel grade No. 1–D may be necessary in extremely cold weather. Fuel Cloud Point The cloud point should be below the lowest ambient temperature at which the engine must start and operate, to prevent the fuel filter elements from plugging with wax crystals. Fuel Viscosity Fuel viscosity must be kept above 1.4 cSt at 100 C) to provide adequate fuel system lubrication. The viscosity also has an effect on the atomization of the fuel when it is injected into the cylinders. F (38