Mercury 3M4Z-8575-A Thermostat Assembly

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

On most models, you can locate the thermostat housing by following the lower radiator hose to the engine. Loosen the hose clamp and detach the hose from the fitting. If necessary, the hose can be cut off or replaced.

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.

It can impact computer function, emissions, and fuel economy.

If the engine runs hot but the lower radiator hose isn't hot, it suggests that the thermostat is stuck closed, preventing coolant flow to the radiator. The thermostat should be replaced.