Mercury 4L5Z-8575-AD Thermostat Assembly

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Check the coolant level, drive belt tension, and temperature gauge before blaming the thermostat. If the engine is hot but the lower radiator hose isn't, replace the stuck closed thermostat. If the hose is hot, further diagnose the cooling system. To replace the thermostat, disconnect the battery and drain the cooling system. Locate the thermostat housing, detach the hose, and replace it if corroded.

If the lower radiator hose is hot, it indicates that the thermostat is open. In this case, further cooling system diagnosis should be made.

It could indicate a possibly stuck open thermostat.

To replace the thermostat, disconnect the negative battery cable, remove the air cleaner air duct if necessary, and drain the cooling system. Locate the thermostat housing by following the upper or lower radiator hose, and disconnect electrical connectors if present. Loosen and detach the hose from the fitting, and if necessary, cut off an old or deteriorated hose and replace it. Remove the housing cover bolts, detach the cover, and remove the old thermostat. Clean the housing and cover surfaces, ensuring a proper seal for the new gasket. Install the new thermostat with the correct orientation, replace the housing cover, and tighten the bolts to specified torque. Reattach the hose, refill the cooling system, reinstall the air cleaner air duct if removed, start the engine, and monitor coolant levels and thermostat operation while checking for leaks.