A hydro-actuated thermostat system for controlling a temperature of an engine, the system comprising: a thermal responsive valve system comprising: a temperature responsive valve having a predefined Start to Open temperature, a thermally actuated piston configured to cause said temperature responsive valve to open when the temperature of a coolant fluid reaches said STO; and a spring, configured to force closing of said temperature responsive valve when the temperature of said coolant fluid is below said STO; and characterized by a hydraulic actuator system comprising: a first control valve; a second control valve; a hydraulic actuator; and a controller configured to control operation of said first and second valves; wherein when said controller induces opening of said first control valve, said hydraulic actuator causes said spring of said thermal responsive valve system to be extended, such that said temperature responsive valve opens at a temperature below said STO; and wherein when said controller induces opening of said second control valve, hydraulic actuator causes said spring of said thermal responsive valve system to be contracted, such that said temperature responsive valve remains closed when the temperature of said coolant fluid reaches said STO. a thermal responsive valve system comprising: a temperature responsive valve having a predefined Start to Open temperature, a thermally actuated piston configured to cause said temperature responsive valve to open when the temperature of a coolant fluid reaches said STO; and a spring, configured to force closing of said temperature responsive valve when the temperature of said coolant fluid is below said STO; and a temperature responsive valve having a predefined Start to Open temperature, a thermally actuated piston configured to cause said temperature responsive valve to open when the temperature of a coolant fluid reaches said STO; and a spring, configured to force closing of said temperature responsive valve when the temperature of said coolant fluid is below said STO; and characterized by a hydraulic actuator system comprising: a first control valve; a second control valve; a hydraulic actuator; and a controller configured to control operation of said first and second valves; wherein when said controller induces opening of said first control valve, said hydraulic actuator causes said spring of said thermal responsive valve system to be extended, such that said temperature responsive valve opens at a temperature below said STO; and wherein when said controller induces opening of said second control valve, hydraulic actuator causes said spring of said thermal responsive valve system to be contracted, such that said temperature responsive valve remains closed when the temperature of said coolant fluid reaches said STO. a first control valve; a second control valve; a hydraulic actuator; and a controller configured to control operation of said first and second valves; wherein when said controller induces opening of said first control valve, said hydraulic actuator causes said spring of said thermal responsive valve system to be extended, such that said temperature responsive valve opens at a temperature below said STO; and wherein when said controller induces opening of said second control valve, hydraulic actuator causes said spring of said thermal responsive valve system to be contracted, such that said temperature responsive valve remains closed when the temperature of said coolant fluid reaches said STO.