Mercury - Thermostat & Seal Kit Cooling System Factory Marine Finish 99155T1. Overview. There's a certain calm that settles in once your engine reaches that perfect running temperature-the moment everything evens out, smooths out, and feels right. Any boater who's spent long days offshore or cruising a familiar stretch of river knows that feeling well. It's the reassurance that your system is protected, that your engine is breathing easy, and that your boat is ready for whatever the day brings.