The pore clogging determination apparatus tentatively determines clogging of a pore (micropore) based on the rate of increase in a coolant temperature at an engine outlet. When the presence of pore clogging has been tentatively determined, the apparatus increases a rotation speed of a coolant pump to determine whether or not the coolant pump is idling. When it has been determined that the coolant pump is idling, the apparatus finalizes the determination of the clogging of the pore (micropore). This suppresses an erroneous determination of clogging of the pore that allows the coolant to flow in an engine cooling system.

Upon a valve rotation angle of a flow rate control valve exceeding a radiator-flow-path closed position during changing of the valve rotation angle of the flow rate control valve in an opening direction of a radiator flow path from a closed state of the radiator flow path, a cooling water starts to circulate through the radiator flow path, and an outlet water temperature or an inlet water temperature of an engine starts to drop. The radiator-flow-path closed position is learned as a valve rotation angle of the flow rate control valve immediately before the outlet water temperature or the inlet water temperature starts to drop during changing of the valve rotation angle of the flow rate control valve in the opening direction of the radiator flow path from the closed state of the radiator flow path.

A leak detection system for a coolant system includes a coolant reservoir including a coolant holding portion configured to store coolant for the coolant system and a gas holding portion, a reservoir inlet fluidically connected to the gas holding portion, a reservoir outlet fluidically connected to the coolant holding portion, and a pressure relief valve including a coolant inlet portion fluidically connected to the coolant holding portion and a pressure relief outlet. The pressure relief valve is configured to respond to pressure in the gas holding portion to vent coolant from the coolant holding portion through the pressure relief outlet without venting gas from the gas holding portion.