Mercury 4L5Z-8575-A Thermostat Assembly

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To replace the thermostat, first disconnect the battery cable and drain the cooling system, saving the coolant if it's in good condition. Then, locate the thermostat housing, loosen the hose clamp, and detach the hose. If the thermostat housing cover is corroded, it should be replaced. For 2.3L and 2.5L engines, disconnect the heater hose and remove the fasteners to access the thermostat. Note its orientation, remove it, and clean the gasket surfaces. Install a new rubber gasket on the thermostat and install it in the housing with the jiggle valve at 12 o'clock. For paper gaskets, wet it with water and align it with the bolt holes. Reattach the housing cover, tighten the bolts, and reconnect the radiator hose. Finally, refill the cooling system, reconnect the battery, and allow the engine to reach normal operating temperature to check for leaks and proper thermostat operation.

Start the engine, monitor for leaks, and ensure proper thermostat operation.

Preliminary checks should include verifying the coolant level, drive belt tension, and temperature gauge operation.

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