How to Change the Thermostat on a Boat

A wise boat owner can extend the life of his investment by changing the boat motor thermostat once a year. Doing it yourself can also save some money. It's a quick task and not difficult to do. Replacing a boat thermostat is very similar to replacing an auto thermostat.

Difficulty: Moderate

Instructions

Things You'll Need:

• Socket wrench set
• Short, flat-blade screwdriver
• New thermostat for your particular motor
• New thermostat housing gasket
• Gasket sealer
• New water hoses or clamps if needed

1. Drain the water from the engine block. This is done by opening one or more of the petcock valves located on the either side of the engine block. The petcocks can be identified by the characteristic "wings" that are grasped and twisted to open. When the water stops draining, you can close the petcock valves. Take care: petcocks are often reverse threaded.

2. Locate the thermostat housing. It is usually located on the top of the engine block near the front. It can be identified by the large water hose connecting it to the water pump on the front of the engine. It will have two or three bolts holding it down. If you have trouble locating the thermostat housing, check with a repair manual or one of the links provided under References.

3. Loosen the hose clamp(s) and remove any water hoses that may be connected to the thermostat housing. This may require sliding a short, flat-blade screwdriver between the hose and the thermostat housing. Twist hoses back and forth to remove.

4. Loosen and remove the bolts holding the thermostat housing to the engine. Take care not to drop any of the lock washers that may be on the bolts. They will drop straight down into the bilge and may be difficult to retrieve.

5. Remove the old thermostat. Note how the thermostat sits in the housing. There is an "up" and a "down" side. This will be important when installing the new thermostat.

6. Scrape the old gasket from both the thermostat housing and the engine block. Use a flat-blade screwdriver to perform this task.

7. Insert the new thermostat in the exact position as the old one. Make sure it sits flat.

8. Add gasket sealer to both sides of the housing gasket and place the gasket on top of the new thermostat. Align the holes to match.

9. Place the thermostat housing on top of the new gasket. Take care to align the holes properly and that the housing fits flush on top of the gasket.

10. Insert the bolts and finger-tighten. Once all the bolts are equally tight, snug them down with the socket wrench. Again, take care that all bolts are evenly set. Failure to do so may crack the thermostat housing.

11. Replace hoses and clamps. Replace any that are damaged or cracked. The task is complete.

Tips & Warnings

• Visit your marine parts store for the proper parts for your boat. Different engines will require different parts.
• Never run the engine, even for the shortest time, when it is out of the water or without having put water into the cooling system. This will surely cause engine damage.

References

• Engine Overheat Troubleshooting
• Keep Your Eye on the Thermostat

Resources

• Mercruiser Parts

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