How to Replace an Outboard Thermostat

If your boat's outboard engine is overheating or won’t come up to temperature like it should, it may simply have a bad thermostat. The difficulty in changing the thermostat varies from model to model, but you won’t find one much easier to replace than a Mercury Tracker 40 two-stroke thermostat.

In this video, we simply move the thermostat from one engine to another, but the process is the same if you’re just replacing yours. Watch the video above or follow the steps below to replace the thermostat in a Mercury outboard engine.

Tools and Parts - Mercury Outboard Thermostat Replacement

  • 13mm socket
  • Phillips head screwdriver
  • Torque wrench

Buy Mercury 40 outboard thermostat

Buy Mercury 40 thermostat housing gasket

Mercury Tracker 40 Thermostat Replacement

Step 1. Use a 13mm socket to remove the four bolts that hold the thermostat cover in place, then remove the cover itself. 

Mercury 40 Tracker two-stroke outboard thermostat replacement

NOTE: You’ll reuse the cover, the spring inside, and the mount that the diaphragm sits on. 

Step 2. Remove the spring and the old thermostat. 

Mercury 40 Tracker outboard thermostat replacement

Mercury 40 outboard thermostat replacement

Mercury outboard thermostat replacement

Step 3. Remove the Philips screw that holds the diaphragm assembly together (found in the cover), then disassemble it (the diaphragm, the mount, and the end cap). 

Mercury outboard thermostat change

Mercury 40 outboard thermostat change

Step 4. Replace the diaphragm and put the assembly back together. 

Mercury 40 Tracker outboard thermostat change

NOTE: If you’re experiencing a lot of resistance as you’re trying to tighten the screw, you may have it mis-threaded, and you could damage the nylon end cap. Back the screw out and try again.

Mercury 40 Tracker two-stroke outboard thermostat change

Step 5. Set the diaphragm assembly into its position in the block. 

How to change an outboard thermostat

NOTE: Just get it into place, the cover will hold it.

Step 6. Place the spring and the thermostat in the cover, then add the gasket and put it all on the motor at once. 

How to replace an outboard thermostat

How to replace a Mercury outboard thermostat

Push the cover into place. It’ll resist because of the spring, but you should be able to push it down onto the block and hand-tighten the bolts. If you can’t push it down flush with the block, something is misaligned and you need to try again. 

How to replace a Mercury 40 outboard thermostat

Step 7. Torque the cover bolts to 16.5 foot-pounds and you’re done. 

How to change a Mercury 40 outboard thermostat

A bad thermostat is just one of several cooling system parts that can cause the overheating of an outboard. Watch the video below to learn more about why outboards overheat.










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