Abstract: Diesel fuel preheaters with a heat exchanger and electric heating serve, for example, at low temperatures to maintain the flowability of diesel fuel and furthermore to quickly heat diesel fuel to achieve defined conditions for a fuel conserving and emission reduced combustion of the fuel and a better engine performance. The diesel fuel preheaters are provided with an integrated control unit and are adaptable to various engines and operational conditions. A special electric heating element in cooperation with the integrated control unit and the heat exchanger provide a fast and economic preheating of the diesel fuel. Furthermore, various advantageous couplings and mechanisms for the regulation of the heat transfer from the heat-deliverying medium such as cooling water to the diesel fuel at the heat exchanger are shown and described.