partial flow of diesel fuel flows through the heat exchanger the effect of heat exchanger which is oversized in order to provide a faster heating action is thus reduced from its previous maximum efficiency to its normal efficiency in which no overheating of the diesel fuel at the outlet 10 occurs the diesel fuel preheater 48 according to fig4 has an immerseable heat exchanger in which the heat transfer may be controlled in a continuously adjustable manner the cup shaped housing 49 of the heat exchanger which may be comprised of cast aluminum is provided at its outer circumference with ribs 50 and at its inner circumference with a smooth cylinder bore 51 a coil 52 made from a heat conducting material such as aluminum band is wound such that it contacts the walls of the cylinder bore 51 with a prestress one end of the coil 52 for example the lower end is fixedly connected to the housing 49 so that no rotation may occur within the coil 52 a tube 53 is provided or in another embodiment the tube 35 of the heating cartridge spaced at a relatively small distance d from the coil the upper end of the coil 52 is fixedly connected to the tube 53 so that no rotation can take place advantageously the coil 52 may be fastened to the tube and the housing by a tongue groove means in a rotation fast manner not shown in the drawing the coil 52 may be made of a flexible heat conducting gauze band for example a metal band in the following paragraph.