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
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.
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.
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).
Step 4. Replace the diaphragm and put the assembly back together.
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.
Step 5. Set the diaphragm assembly into its position in the block.
NOTE: Just get it into place, the cover will hold it.
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.
Step 7. Torque the cover bolts to 16.5 foot-pounds and you’re done.