Mercury - Thermostat EFI FourStroke Cooling System Marine Alloy 8M0186873. Overview. When you spend your weekends chasing early light on quiet water or easing away from the dock with the family, you count on your engine to settle into that smooth, steady rhythm you know so well. A dependable thermostat is a small part, but it's the reason your Mercury FourStroke warms up cleanly, runs efficiently, and never leaves you guessing. The Quicksilver 8M0186873 Thermostat gives you that peace of mind-th