Thermostat failure judgment device

A failure judgment device for a thermostat arranged in a cooling-water circulation passage in an engine, comprising a water-temperature sensor for detecting a temperature of cooling water having passed through the engine and a controller for monitoring rise in temperature of cooling water at cold startup through a detection signal inputted from said water-temperature sensor and for calculating predicted water temperature of the engine as from beginning of the startup to judge failure of the thermostat when said predicted temperature reaches a valve opening temperature of the thermostat with the temperature of the cooling water being not beyond a threshold value, said controller calculating all together a heat quantity transferring from respective cylinders in said engine to the cooling water on the basis of information on a revolution speed of the engine and a fuel injection amount, and a heat loss of the cooling water cooled in contact with ambient air on the basis of information on the revolution speed of the engine, a car speed, an intake air temperature and a temperature of the cooling water, determining an eventual heat quantity obtained by the cooling water through subtraction of the heat loss of the cooling water cooled, said determined heat quantity being converted to raised temperature of the cooling water which in turn is added to an actually measured value of the cooling water at the beginning of the startup to determine the predicted water temperature.