Failure detection apparatus for cooling system of internal combustion engine

A failure detection apparatus for a cooling system of an internal combustion engine is disclosed. The cooling system has a radiator, a cooling water pipe for circulating a cooling water in the engine to the radiator, and a thermostat provided in the cooling water pipe for opening and closing the cooling water pipe. An estimated cooling water temperature of the engine is calculated according to the engine operating condition. The estimated cooling water temperature is compared with a determination threshold set according to the detected vehicle speed at the time the detected cooling water temperature has exceeded a predetermined water temperature. If the estimated cooling water temperature is lower than or equal to the determination threshold, it is determined that the thermostat is normal.