Mercury – Thermostat | Cooling System Component | Marine Alloy | 8M0139434Mercury Thermostat Cooling System Component Marine Alloy 8M0139434 Overview Anyone who spends real time on the water knows how important a steady running outboard is. You feel it in the throttle response, in the smooth hum at idle, and in that confidence you get when you point the bow toward open water without wondering what might go wrong. The thermostat is one of those small parts quietly keeping everything in balance, and Mercurys 8M0139434