is emitted from the diesel injector thereby increasing the fuel 39 s ability to enter into more complete combustion with the engine thus improving fuel economy in operating the system in a vehicle when the vehicle switch is turned on the relay coil 35 is energized permitting contact switch 40 to close and permitting heater dc current to flow to the power transistor 39 when the thermister sensor 32 senses a less than optimum temperature in the fuel line circuit 38 command actuates the power transistor 39 to close the heater circuit with resultant dc current flow from battery 36 through heating element 41 in heat exchange unit 10 and through contact 40 to power transistor 39 and to ground 44 heat from tubular heating element 41 flows through the heat exchanger 39 s cylinder 10 directly into the fuel carried in tubes 12 12 when the fuel line thermister sensor 32 senses a one half degree farenheit fuel temperature above the set optimum temperature the circuit 38 command actuates the power transistor 39 to open the heater circuit interrupting the flow of dc current through heating element 41 and flow of heat through heat exchange cylinder 10 into the fuel carried in tubes 12 12 thus the system cycles on and off in response to fuel line temperature maintenance as measured at the entrance to the engine.