{"product_id":"mercury-quicksilver-thermostat-8m0139434-125f-15-20hp-efi-fourstroke","title":"Mercury Quicksilver Thermostat 8M0139434 — 125°F | 15 \u0026 20 HP EFI FourStroke","description":"\u003ch2\u003eMercury Quicksilver Thermostat — OEM 8M0139434 | 125°F | 15 \u0026amp; 20 HP EFI FourStroke\u003c\/h2\u003e\u003cp\u003eThis genuine OEM thermostat regulates cooling-water flow on selected Mercury 15 HP and 20 HP EFI FourStroke outboards, helping the engine reach and hold the correct operating temperature for efficient combustion and long-term reliability. The 8M0139434 uses a self-cleaning design that allows cooling-system particles to pass through the closing surfaces rather than accumulate — an important feature in saltwater and silty environments. Rated at 125°F, it is specific to the 2-cylinder EFI configuration and serial range 0R833820 and above.\u003c\/p\u003e\u003ch2\u003ePart Information\u003c\/h2\u003e\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eOEM Part Number:\u003c\/strong\u003e 8M0139434\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eBrand:\u003c\/strong\u003e Mercury \/ Quicksilver\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eOfficial Product Type:\u003c\/strong\u003e Engine cooling-system thermostat\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eTemperature Rating:\u003c\/strong\u003e 125°F\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eEngine Configuration:\u003c\/strong\u003e 2-cylinder EFI FourStroke\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eSerial Range:\u003c\/strong\u003e 0R833820 and above\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eDesign:\u003c\/strong\u003e Self-cleaning thermostat\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eSystem:\u003c\/strong\u003e Engine cooling system\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eGTIN \/ UPC:\u003c\/strong\u003e 745061050396\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch2\u003eCompatible Applications\u003c\/h2\u003e\u003cul\u003e\n\u003cli\u003eMercury 15 HP EFI FourStroke — 2-cylinder, serial 0R833820 and above (2017 and newer)\u003c\/li\u003e\n\u003cli\u003eMercury 20 HP EFI FourStroke — 2-cylinder, serial 0R833820 and above (2017 and newer)\u003c\/li\u003e\n\u003cli\u003eConfirm engine horsepower, cylinder count, and serial number before ordering — earlier 15\/20 HP FourStroke engines below serial 0R833820 may use a different thermostat\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch2\u003eRelated Parts\u003c\/h2\u003e\u003cul\u003e\n\u003cli\u003e\u003ca href=\"\/products\/mercury-outboard-thermostat-assembly-120f-8m0090819\"\u003eMercury Outboard Thermostat Assembly 120°F — 8M0090819\u003c\/a\u003e\u003c\/li\u003e\n\u003cli\u003e\u003ca href=\"\/products\/mercury-outboard-140f-thermostat-8m0119207\"\u003eMercury Outboard Thermostat 140°F — 8M0119207\u003c\/a\u003e\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch2\u003eWhen to Replace\u003c\/h2\u003e\u003cul\u003e\n\u003cli\u003eEngine runs consistently below normal operating temperature (thermostat stuck open)\u003c\/li\u003e\n\u003cli\u003eEngine overheats and cooling-system diagnosis points to a faulty thermostat (stuck closed)\u003c\/li\u003e\n\u003cli\u003eThermostat fails to open or close correctly on bench testing\u003c\/li\u003e\n\u003cli\u003eAs part of a scheduled cooling-system service or when the thermostat housing is opened for any reason\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch2\u003eService Interval\u003c\/h2\u003e\u003cp\u003eInspect the thermostat annually or whenever the cooling system is serviced. Replace immediately if the thermostat fails to open or close at the correct temperature on testing. Many technicians recommend replacing the thermostat any time the thermostat housing is removed, as reinstalling a used thermostat is a leading cause of repeat cooling-system issues.\u003c\/p\u003e\u003ch2\u003eInstallation Notes\u003c\/h2\u003e\u003cul\u003e\n\u003cli\u003eAllow the engine to cool fully before servicing the cooling system\u003c\/li\u003e\n\u003cli\u003eRemove the thermostat cover and inspect the thermostat cavity and sealing surfaces for corrosion or debris\u003c\/li\u003e\n\u003cli\u003eInstall the replacement thermostat in the correct orientation per the Mercury service procedure\u003c\/li\u003e\n\u003cli\u003eUse a new gasket or seal on reassembly if specified\u003c\/li\u003e\n\u003cli\u003eRun the engine and verify normal operating temperature after installation\u003c\/li\u003e\n\u003c\/ul\u003e\u003ch2\u003eTechnical FAQ\u003c\/h2\u003e\u003cp\u003e\u003cstrong\u003eWhy does the temperature rating matter?\u003c\/strong\u003e\u003cbr\u003eMercury uses different thermostat temperatures across its outboard lineup — 120°F, 125°F, 130°F, and 140°F ratings are all used on different engines. Installing the wrong temperature thermostat will cause the engine to run outside its designed operating range. Always match the temperature rating and part number to your specific engine.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eWhat makes this a “self-cleaning” thermostat?\u003c\/strong\u003e\u003cbr\u003eThe self-cleaning design allows small particles and cooling-system debris to pass through the thermostat’s closing surfaces rather than build up on the seat. This reduces the risk of the thermostat sticking due to debris accumulation, which is particularly valuable in saltwater or silty operating environments.\u003c\/p\u003e\u003cp\u003e\u003cstrong\u003eIs this thermostat compatible with older 15\/20 HP FourStroke engines?\u003c\/strong\u003e\u003cbr\u003eNo. 8M0139434 is specified for serial 0R833820 and above (2017 and newer). Earlier engines use a different thermostat configuration. Always confirm the serial number before ordering.\u003c\/p\u003e\u003ch2\u003eParts Desk Insights\u003c\/h2\u003e\u003cp\u003eThe most important fitment details for this thermostat are the 125°F rating and serial 0R833820 and above — these distinguish it from other Mercury outboard thermostats used on different engine families and configurations. A thermostat stuck open is one of the most common causes of an engine that never fully warms up or runs poorly at low speed, and it’s often overlooked because the engine doesn’t overheat. If a customer reports poor fuel economy or rough low-speed running on a 15\/20 HP EFI, a stuck-open thermostat is worth checking before diving into fuel-system diagnosis. Browse the full \u003ca href=\"\/collections\/mercury-mercruiser-and-quicksilver-cooling-system-parts\"\u003eMercury Cooling System Parts\u003c\/a\u003e collection for related thermostats, housings, and water pump components.\u003c\/p\u003e","brand":"MERC","offers":[{"title":"Default Title","offer_id":51324694036516,"sku":"8M0139434","price":53.32,"currency_code":"CAD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0743\/5799\/7604\/files\/6b2f51fa-a345-4b4b-82b7-4077c187ee5e.png?v=1788213412","url":"https:\/\/marisolmarine.com\/products\/mercury-quicksilver-thermostat-8m0139434-125f-15-20hp-efi-fourstroke","provider":"Marisol Marine Company","version":"1.0","type":"link"}