To prevent misjudgment on failure of thermostat when temperature of cooling water rises only slowly or rises rapidly.
This diagnosis device judges the followings as valve-opening failure when actual temperature of cooling water detected by water temperature sensor S1 is lower than that of specified valve-opening temperature of thermostat 5, after elapsing of high-load, high-speed operation for more than specified time. Or, it judges finally to be valve-opening failure when either the first failure judgment measure that temporarily judges valve-opening failure based on actual cooling water temperature by water temperature sensor S1, or the second failure judgement measure that temporarily judges as valve-opening failure when heat value ratio R=Qorh/Qig is bigger than the threshold value, after radiation volume Qorh from radiator 3 is calculated based on anticipated cooling water temperature according to operation of engine, and, on the other hand, heat intake volume Qig of cooling water from engine is calculated, judges as failure.
HOSOGAI TETSUSHI
MAKIMOTO SEIJI
