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In an automatic internal combustion engine fire protective system, the combination with the engine of an indicator operable by the continuous flow of a fluid, means on the engine to maintain a degree of pressure different from atmospheric pressure during the time of operation of the engine, a normally closed fluid system including the indicator and the pressure maintaining means, and means for opening the fluid system to the atmosphere upon the outbreak of fire, said indicator comprising a body member formed with a chamber, inlet and outlet connections for connecting said chamber in said normally closed fluid system and for permitting a flow of fluid through the chamber, a movable element in said chamber adapted to remain normally motionless but to move continuously upon the creation of a flow of fluid through the chamber, and a sight opening for permitting observation of the movable element in either its motionless or its moving condition.