Outboard motors require a flow of cooling water to maintain proper operating temperature. At high speeds maximum cooling is needed to prevent engine damage while at idle the engine must stay warm enough to avoid fouling the spark plugs. The thermostat maintains the correct temperature. The constant flow of water through the thermostat means that it should be replaced regularly. The Evinrude V4 cross-flow engine uses two thermostats, one for each cylinder head.
Remove the engine cover and locate the thermostat housing at the rear of the engine. It will have two hoses, one leading to each cylinder head. Loosen the hose clamps with a screwdriver and remove the hoses. Inspect the hoses and replace them if they are cracked or worn. The thermostat housing is held in place by three screws. Use a screwdriver to remove these and carefully separate the housing from the adaptor plate.
Use a screwdriver to remove the three small screws that hold the cover onto the valve body. Pull out the springs, thermostats, relief valves and all grommets with your fingers (or pliers if they are stuck) and discard them. Clean any corrosion from the valve body with emery cloth or fine sandpaper, particularly if the engine has been operated in salt water.
Install new grommets into the valve body. Install the two new relief valves into the grommets (the smaller, shorter side of the relief valve should face away from the grommet). Position each new thermostat into the valve body, with the temperature-sensing end pointing into the valve body. Insert the new relief valve springs into the bores in the housing.
Coat the outside of the valve body and a new gasket with gasket sealing compound. Put the gasket in place and replace the cover onto the valve body. Install the valve body into the adaptor housing, making sure that the relief valves fit into the relief valve springs. Secure the valve body in place with the three screws.
Secure the adaptor housing to the engine adaptor plate with the three screws. Replace the hoses between the thermostat housing and the cylinder heads and secure them with hose clamps. Run the engine with water flowing through it to check for any leaks.