Diesel fuel contains wax that crystallizes at low temperatures. Below approximately 15°F, the wax forms gel-like crystals that clog fuel filters and starve the engine of fuel.
Fuel stations in cold-weather markets switch to 'winter blend' diesel automatically each fall — typically a mix of #1 and #2 diesel that resists gelling down to -10°F or lower.
The problem: drivers from warm regions traveling to cold regions in winter may have full tanks of summer-blend diesel that gels in the cold. Same problem for diesel pickups parked all summer that drivers fill before a winter trip.
Anti-gel additives ($10–$15 per bottle) added to the tank at fill-up prevent the problem. Pour in before adding fuel for proper mixing.
If you've already gelled, options are limited: tow to a heated shop where the gel can thaw, add anti-gel and wait (slow process), or replace the clogged fuel filter and add anti-gel.