A thermostatic valve assembly for a cooling system of an internal combustion engine provided with a radiator is provided. The thermostatic valve assembly includes a valve body having an inlet from the radiator, an inlet from the engine and an outlet to the engine. A main valve plate opens and closes the pathway of a cooling fluid from the radiator. A wax element is placed in a chamber of the valve body in fluid communication with the inlets and the outlet. The wax element has a piston mechanically connected to the main valve plate at a distal end. A main spring keeps the main valve plate closed until a target temperature of the cooling fluid coming from the engine is reached. An auxiliary valve plate opens and closes a direct pathway of the cooling fluid from the radiator to the chamber in which the wax element is placed.