Mercury 3L3Z-8575-AC Thermostat Assembly

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You should check the coolant level, drive belt tension, and temperature gauge (or light) operation before diagnosing a faulty thermostat. If your engine takes too long to warm up, this means there is a stuck open thermostat that has to be replaced. When an upper radiator hose remains cool while the engine runs hot then it is very likely that its thermostat is stuck closed hence it needs replacement. When you feel a hot upper radiator hose it means that the thermostat is open so you need to find out the reasons behind the engine heat-up. Overheating results in serious problems such as head gasket leaks, scuffed pistons as well as warped or cracked cylinder heads. You should drain the coolant, remove the distributor cap (if necessary), disconnect hoses, remove the housing cover, extract the old thermostat, clean surfaces, apply sealant or insert O-ring seal, install a new thermostat, attach cover, tighten bolts, reconnect hoses (if they have been disconnected), refill coolant and check for leaks when replacing the thermostat.

Don't drive without a thermostat as it can affect emissions and fuel economy. Before blaming the thermostat, check coolant level, drivebelt tension, and temperature gauge. If the engine takes long to warm up or runs hot without the upper radiator hose being hot, the thermostat is likely faulty and needs replacement. Overheating can cause engine damage. To replace the thermostat, drain coolant below the thermostat housing level, disconnect the upper radiator hose, remove the cover and O-ring seal, and note the thermostat's installation. Install a new thermostat with the correct opening temperature, clean the mating surfaces, and apply RTV sealant to a new gasket or O-ring seal. Reattach the cover, radiator hose, refill the system, check for leaks, and repeat steps if necessary.

If the upper radiator hose feels hot, it indicates that the thermostat is open. In this case, further investigation into overheating issues should be done.

If the engine runs hot and the upper radiator hose isn't hot, it indicates that the thermostat is probably stuck shut and should be replaced.