Engine cooling device

An engine cooling device comprising:

• a plurality of flow paths provided in parallel as flow paths for an engine coolant that circulates from an outlet of a water jacket provided inside an engine to an inlet of the water jacket, the flow paths including a radiator water passage provided through a radiator, a temperature raising water passage provided with a temperature raising heat exchanger for raising a temperature of another fluid with heat of the engine coolant, and a bypass water passage through which the engine coolant flows while the engine coolant bypasses the radiator and the temperature raising heat exchanger;
• a thermostat configured to open and close the radiator water passage;
• a switching valve configured to switch between a state in which flow of the engine coolant from the temperature raising water passage to the water jacket is prohibited and a state in which the flow of the engine coolant from the temperature raising water passage to the water jacket is allowed;
• and
• a diagnosis device configured to perform an abnormality diagnosis of the thermostat based on a comparison result between a rise rate of an outlet water temperature that is a temperature of the engine coolant flowing out of the water jacket and a determination value,
• wherein the diagnosis device is configured to set, during a transition period that is a period until a predetermined time elapses after the switching valve switches from the state in which the flow of the engine coolant from the temperature raising water passage to the water jacket is prohibited to the state in which the flow of the engine coolant from the temperature raising water passage to the water jacket is allowed, a value smaller than a value before a start of the transition period as the determination value to perform the abnormality diagnosis.