A cooling system for an engine is provided. The cooling system includes coolant flow paths including a first flow path and a second flow path and where coolant circulates, a coolant pump for circulating the coolant within the coolant flow paths, a flow rate control valve for adjusting a flow rate of the coolant through the second flow path, a temperature detector for detecting a temperature of the coolant within the first flow path, a valve controller for adjusting an opening of the flow rate control valve based on the temperature detected by the temperature detector, and an output level determiner for determining an output level of the engine based on at least one of a fuel injection amount for the engine and an engine speed.