Mercury - 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 Mercury's 8M0139434 Thermostat does that job with the kind of relia