A fuel system for a diesel truck has a first fuel tank (80) and a second fuel tank (82). A siphon line (90) is open to the lower portions of the tanks and extends above the upper portions of both tanks. A manually operated priming pump (98) is used to remove air from the siphon line. Fuel is delivered from the first tank to the engine of the truck through a fluid delivery line (84). As fuel is drawn from the first tank, the fuel levels in the tanks equalize as the result of flow through the siphon line. Fuel not consumed by the engine is returned through an overflow line (86). The returning fuel is warmed as a result absorbing energy during pumping and having been in close proximity to the engine. A three-way control valve (88) directs the warm returning fuel to either the first or second fuel tank in response to the temperature sensed by a temperature sensor (94). In winter, the system operates to keep the fuel delivered from the first tank to the engine warm, and in summer prevents the fuel from becoming too hot.