Engine cooling device

An engine cooling device in which, 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, a radiator water passage provided through a radiator, a warm-up water passage provided with a warm-up heat exchanger for warming up 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 warm-up heat exchanger are provided in parallel, the engine cooling device including a thermostat configured to open and close the radiator water passage, and a switching valve configured to switch between a state in which flow of the engine coolant from the warm-up water passage to the water jacket is prohibited and a state in which the flow of the engine coolant from the warm-up water passage to the water jacket is allowed, the engine cooling device comprising:

• 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 difference between a temperature of the engine coolant flowing out of the warm-up heat exchanger and the outlet water temperature decreases below a predetermined value after the switching valve switches from the state in which the flow of the engine coolant from the warm-up water passage to the water jacket is prohibited to the state in which the flow of the engine coolant from the warm-up water passage to the water jacket is allowed, a value smaller than a value during a period other than the transition period as the determination value to perform the abnormality diagnosis.