Abstract: The invention relates to a fuel economizer preheating device operating by recovering energy from the fuel itself in the supply circuit of an engine, in particular but not exclusively a diesel engine. Part of the heat supplied to the fuel between the tank and the injectors is transferred by the excess fuel, not to the tank but upstream of the injection pump to the fuel arriving in this region.The device is characterized by the fact that it comprises a heat exchanger (10) removing heat absorbed by the fuel in the vicinity of the injection pump or upstream of the latter and transferring the heat to the fuel arriving upstream of the injection pump (4).Preferably, the exchanger (10) is located upstream of a filter (5) in the immediate vicinity of the latter.A branch pipe (11), possibly integrated in the exchanger (10) makes it possible to by-pass the exchanger, depending on the temperature of the fuel.