It’s not your imagination—modern diesel engines burning ultra-
low-sulfur diesel (ULSD) fuel "gel" more easily on cold winter
days than diesel engines built 20 or 30 years ago. Gelling
occurs when wax, a necessary component of No. 2 diesel fuel,
crystallizes at low temperatures. The "cloud point" of a
particular diesel fuel is the temperature at which crystals of
wax begin to form. The "gel point" is the temperature at which
enough wax crystals develop to transform the fuel from liquid to
semi-solid. The gel point for No. 2 diesel varies but ranges
between 10°F and 20°F. Most diesel fuel now sold in the U.S. is
ULSD to meet the needs of Environmental Protection
Agency–mandated Tier III and newer diesel engines. The problem
with these engines is that their precise, computer-controlled
fuel injectors require fuel filtered to as little as 2 microns.
Wax crystals in chilled diesel fuel measure 50 to 250 microns.
As soon as temperatures drop to the cloud point in a 2-micron
fuel filter, wax crystals collect and res