Cooling system of an internal combustion engine comprising a cooling circuit with - a pump to circulate coolant, - a radiator suitable to disperse heat produced by the internal combustion engine in the ambient, - at least a first pipe to convey coolant from the internal combustion engine towards the radiator and - at least a second pipe connecting the radiator with the pump, The system further comprising - a cylinder /piston assembly defining a chamber suitable to be hydraulically connected with said second pipe though a third pipe communicating with said second pipe, - an actuator connected to said piston in order to control its displacement in said cylinder, by varying said chamber volume, - a pressure sensor in hydraulic communication with said cooling circuit and - an electronic control unit coupled with said pressure sensor and with said actuator and configured to control said actuator in order to maintain a pressure measured by said pressure sensor above a predetermined threshold; the system being characterized in comprising a three-way valve, wherein a first port of the three-way valve communicates with said cooling circuit, wherein said cylinder/piston assembly is connected to a second port of said three-way valve, and wherein the system further comprising a coolant tank connected with a third port of said three-way valve; wherein according to a first condition a) the three-way valve connects the cylinder/piston assembly with said coolant tank, and according to a second condition b) the three-way valve connects the cylinder/piston assembly with the cooling circuit and wherein said control unit is configured to control also said three-way valve so as the three-way valve is in said b) condition when the piston displacement is adjusted to maintain a pressure measured by said pressure sensor above said predetermined threshold. - a pump to circulate coolant, - a radiator suitable to disperse heat produced by the internal combustion engine in the ambient, - at least a first pipe to convey coolant from the internal combustion engine towards the radiator and - at least a second pipe connecting the radiator with the pump, The system further comprising - a cylinder /piston assembly defining a chamber suitable to be hydraulically connected with said second pipe though a third pipe communicating with said second pipe, - an actuator connected to said piston in order to control its displacement in said cylinder, by varying said chamber volume, - a pressure sensor in hydraulic communication with said cooling circuit and - an electronic control unit coupled with said pressure sensor and with said actuator and configured to control said actuator in order to maintain a pressure measured by said pressure sensor above a predetermined threshold; the system being characterized in comprising a three-way valve, wherein a first port of the three-way valve communicates with said cooling circuit, wherein said cylinder/piston assembly is connected to a second port of said three-way valve, and wherein the system further comprising a coolant tank connected with a third port of said three-way valve; wherein according to a first condition a) the three-way valve connects the cylinder/piston assembly with said coolant tank, and according to a second condition b) the three-way valve connects the cylinder/piston assembly with the cooling circuit and wherein said control unit is configured to control also said three-way valve so as the three-way valve is in said b) condition when the piston displacement is adjusted to maintain a pressure measured by said pressure sensor above said predetermined threshold.