Mercury - 170 Degree Thermostat Cooling System Component Brass & Stainless Steel 8M0045990. Overview. There's a certain comfort that comes from knowing your engine is running at the temperature it was born to run at. When you're easing out of the no-wake zone or pushing across open water with a full day ahead, you don't want to wonder whether your cooling system is keeping up. A reliable thermostat is one of those quiet heroes-rarely seen, hardly ever talked about, but always working in the back