Mercury - Thermostat Cooling System Factory Marine Finish 991551. Overview. There's a certain comfort in watching your engine settle into that perfect operating temperature-steady, calm, and exactly where it should be. It's the feeling you get when you've been on the water long enough to know when something is right, and when something isn't. A good thermostat doesn't shout for attention; it simply keeps the heart of your boat running smooth and predictable. That quiet reliability is why so many