Mercruiser Thermostat - 18-3549 Mercruiser Thermostat - 18-3549 by Sierra. Please use the Red PARTS CHARTS button (top of any page) to locate the correct part for your application. $ 19.08               More Information      Quantity:     Mercury/Mariner Thermostat - 18-3549 Mercury/Mariner Thermostat - 18-3549. Please use the Red PARTS CHARTS button (top of any page) to locate the correct part for your application. All prices are per unit (50' Coil=50 Units, 2 PK=2 Units, and etc.). Verify your quantity is correct at Check-Out and adju as necessary. Send us an email (Contact Us button at top of any page) if you have any questions. $ 19.08               More Information      Quantity: