Abstract: An improved system for controlling the temperature of fuel entering an engine, the system including a heat exchanger interposed between a fuel supply and the engine for preheating the fuel. The heat exchanger is formed by an exterior, fluid-tight casing having a plurality of fluid-tight tubes housed therein to isolate fluid passing through the tubes from the remainder of the interior of the casing so as to provide two flow paths, one through the tubes, and the other through the remainder of the casing, and having fuel and engine-coolant fluid, inlet and outlet ports. The improvement including an engine-coolant fluid flow control valve positioned adjacent the fuel inlet port, a fuel temperature sensor positioned adjacent the fuel outlet port and a fluid valve-actuating relay operative to receive fuel temperature information from the temperature sensor and control the fluid flow valve.