How to Replace a MerCruiser Alpha Thermostat

by Jared Curtis

The thermostat in your MerCruiser Alpha monitors the temperature of the engine. The impeller inside the out-drive sucks water into the engine. If the engine temperature is too hot, the thermostat opens to allow more water into the engine. If the engine is too cold, the thermostat closes so that the water cannot enter the engine to cool it. Replacing the thermostat in your Mercruiser Alpha is definitely a DIY project, and you can find a new one at your local boat shop.

Step 1

Open the engine cover of your MerCruiser to access the engine. Locate the thermostat housing. It mounts to the top of the engine near the front. There are four hoses that connect to the housing and are attached with hose clamps.

Step 2

Remove the two bolts securing the housing to the engine block. Pull the cover off the thermostat housing. Remove the gasket.

Step 3

Make note where the indexing notch or the wobble valve is positioned. Grasp the thermostat inside the housing with pliers. Pull the thermostat out of the housing and discard. Clean the mating surfaces to remove all traces of gasket material.

Step 4

Position the new thermostat according the indexing notch or wobble valve. Place the new thermostat in the housing. Apply the new gasket between the cover and the thermostat housing.

Step 5

Install the cover on the housing. Align the holes of the cover, gasket and housing and install the bolts. Tighten them, using a ratchet and socket.

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