{"product_id":"18-3638-outboard-thermostat-for-mercury-mariner","title":"18-3638 Outboard Thermostat for Mercury Mariner | JLM Marine","description":"\u003ch2\u003eThermostat for Mercury Mariner Outboard – Fits 2.5-6 HP Models\u003c\/h2\u003e\n\n\u003cp\u003eThis high-quality aftermarket thermostat is engineered to fit Mercury Mariner 2-stroke outboard engines ranging from 2.5 HP to 6 HP. Designed as a direct replacement for OEM part numbers 804540 and 853704, it ensures reliable temperature regulation for your outboard motor.\u003c\/p\u003e\n\n\u003cp\u003eThe kit includes both the thermostat and a rubber seal ring, providing everything needed for a complete installation. Compatible with Mercury Mariner 2.5-3.5 HP models (serial 0R131842 \u0026amp; Up) and 4-6 HP models (serial 0R411838 \u0026amp; Up).\u003c\/p\u003e\n\n\u003cul\u003e\n\u003cli\u003eComplete kit includes thermostat and rubber seal ring\u003c\/li\u003e\n\u003cli\u003eDirect replacement for OEM 804540 and 853704\u003c\/li\u003e\n\u003cli\u003eEnsures proper engine temperature control\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003ePart Number Cross-Reference\u003c\/h2\u003e\n\n\u003cp\u003eUse this table to confirm equivalent part numbers across OEM and aftermarket brands.\u003c\/p\u003e\n\n\u003ctable border=\"1\" cellpadding=\"8\" cellspacing=\"0\"\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth\u003eBrand\u003c\/th\u003e\n\u003cth\u003ePart Number\u003c\/th\u003e\n\u003c\/tr\u003e\n\u003c\/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd\u003eMercury Mariner\u003c\/td\u003e\n\u003ctd\u003e804540\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eMercury Mariner\u003c\/td\u003e\n\u003ctd\u003e853704\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eJLM Marine\u003c\/td\u003e\n\u003ctd\u003e18-3638\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003c\/tbody\u003e\n\u003c\/table\u003e\n\n\u003ch2\u003eCompatible Engine Models\u003c\/h2\u003e\n\n\u003cp\u003eConfirm your engine model\/year in the table below. For Mercury owners, here's a guide to help you identify your engine's serial\/model: \u003ca href=\"https:\/\/jlmmarine.com\/blogs\/serial-number-guide\/mercury-outboard-serial-number-guide\" rel=\"nofollow noopener\" target=\"_blank\"\u003eMercury Serial Number Guide\u003c\/a\u003e. You can also review exploded diagrams to verify part numbers: \u003ca href=\"https:\/\/jlmmarine.com\/blogs\/diagrams\/mercury-parts-catalog\" rel=\"nofollow noopener\" target=\"_blank\"\u003eMercury Parts Diagrams \u0026amp; Catalog\u003c\/a\u003e.\u003c\/p\u003e\n\n\u003cdiv style=\"overflow-x:auto;\"\u003e\n\u003ctable border=\"1\" cellpadding=\"8\" cellspacing=\"0\"\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth\u003eBrand\u003c\/th\u003e\n\u003cth\u003eModel\u003c\/th\u003e\n\u003cth\u003eRated HP\u003c\/th\u003e\n\u003cth\u003eNotes\u003c\/th\u003e\n\u003c\/tr\u003e\n\u003c\/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd\u003eMercury Mariner\u003c\/td\u003e\n\u003ctd\u003eSerial 0R131842 \u0026amp; Up\u003c\/td\u003e\n\u003ctd\u003e2.5-3.5 HP\u003c\/td\u003e\n\u003ctd\u003e2-Stroke Outboards\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eMercury Mariner\u003c\/td\u003e\n\u003ctd\u003eSerial 0R411838 \u0026amp; Up\u003c\/td\u003e\n\u003ctd\u003e4-6 HP\u003c\/td\u003e\n\u003ctd\u003e2-Stroke Outboards\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003c\/tbody\u003e\n\u003c\/table\u003e\n\u003c\/div\u003e\n\n\u003ch2\u003eTechnical Specifications\u003c\/h2\u003e\n\n\u003ctable border=\"1\" cellpadding=\"8\" cellspacing=\"0\"\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth\u003eSpecification\u003c\/th\u003e\n\u003cth\u003eValue\u003c\/th\u003e\n\u003c\/tr\u003e\n\u003c\/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd\u003eOEM Number\u003c\/td\u003e\n\u003ctd\u003e18-3638\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eReplaces OEM Part Numbers\u003c\/td\u003e\n\u003ctd\u003e804540, 853704\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eApplication\u003c\/td\u003e\n\u003ctd\u003eMercury Mariner 2.5-6 HP Outboards\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003ctr\u003e\n\u003ctd\u003eEngine Type\u003c\/td\u003e\n\u003ctd\u003e2-Stroke\u003c\/td\u003e\n\u003c\/tr\u003e\n\u003c\/tbody\u003e\n\u003c\/table\u003e\n\n\u003ch2\u003ePackage Contents\u003c\/h2\u003e\n\n\u003cul\u003e\n\u003cli\u003e\u003cspan style=\"color:red;\"\u003e\u003cstrong\u003e1 × Thermostat\u003c\/strong\u003e\u003c\/span\u003e\u003c\/li\u003e\n\u003cli\u003e\u003cspan style=\"color:red;\"\u003e\u003cstrong\u003e1 × Ring Rubber Seal\u003c\/strong\u003e\u003c\/span\u003e\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eInstallation Guide\u003c\/h2\u003e\n\n\u003col\u003e\n\u003cli\u003eAllow the engine to cool completely before beginning work. Ensure the ignition is off and disconnect the battery if applicable.\u003c\/li\u003e\n\u003cli\u003eLocate the thermostat housing on your Mercury Mariner outboard engine. It is typically found near the cylinder head or water jacket outlet.\u003c\/li\u003e\n\u003cli\u003eRemove the mounting bolts securing the thermostat housing cover using the appropriate wrench or socket. Keep track of all fasteners.\u003c\/li\u003e\n\u003cli\u003eCarefully lift the housing cover and remove the old thermostat and seal. Inspect the housing for corrosion or debris and clean thoroughly.\u003c\/li\u003e\n\u003cli\u003eInstall the new rubber seal ring into the groove of the thermostat housing. Ensure it seats properly without twisting or pinching.\u003c\/li\u003e\n\u003cli\u003ePosition the new thermostat in the housing with the temperature-sensing element facing the correct direction (typically toward the engine block).\u003c\/li\u003e\n\u003cli\u003eReplace the housing cover and hand-tighten the mounting bolts. Then torque bolts to manufacturer specifications in a crisscross pattern to ensure even sealing.\u003c\/li\u003e\n\u003cli\u003eReconnect the battery if disconnected. Start the engine and allow it to reach operating temperature while monitoring for leaks around the thermostat housing.\u003c\/li\u003e\n\u003cli\u003eCheck coolant levels and top off if necessary. Verify that the engine temperature remains within normal operating range.\u003c\/li\u003e\n\u003c\/ol\u003e\n\n\u003ch2\u003eTroubleshooting \u0026amp; FAQ\u003c\/h2\u003e\n\n\u003cp\u003e\u003cstrong\u003eQ: How do I confirm this thermostat fits my Mercury Mariner outboard?\u003c\/strong\u003e\u003cbr\u003e\nA: Check your engine's serial number plate. This thermostat fits 2.5-3.5 HP models with serial 0R131842 \u0026amp; up, and 4-6 HP models with serial 0R411838 \u0026amp; up. Cross-reference your current part number with OEM 804540 or 853704.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eQ: What are the signs that my thermostat needs replacement?\u003c\/strong\u003e\u003cbr\u003e\nA: Common symptoms include engine overheating, failure to reach operating temperature, excessive steam from the cooling system, or coolant leaks around the thermostat housing. If your temperature gauge reads abnormally high or low, inspect the thermostat.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eQ: How often should I replace the thermostat on my outboard?\u003c\/strong\u003e\u003cbr\u003e\nA: Thermostats should be inspected annually and replaced every 2-3 years or sooner if symptoms of failure appear. Always replace the rubber seal when installing a new thermostat to prevent leaks.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eQ: Does this kit include the gasket or seal?\u003c\/strong\u003e\u003cbr\u003e\nA: Yes, this package includes both the thermostat and the rubber seal ring required for proper installation. No additional seals need to be purchased separately.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eQ: Is this an OEM part or aftermarket?\u003c\/strong\u003e\u003cbr\u003e\nA: This listing is an aftermarket replacement part (not an OEM component). We build to OEM specifications using comparable-grade materials. Production is done in an ISO9002-certified facility, and each batch is inspected for consistent performance.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eQ: How fast do you ship and how much does it cost?\u003c\/strong\u003e\u003cbr\u003e\nA: We offer fast global shipping—typically 5–15 days worldwide—and standard shipping is free. Expedited options are available at checkout depending on your location.\u003c\/p\u003e\n\n\u003cp\u003e\u003cstrong\u003eQ: What is your warranty and return policy?\u003c\/strong\u003e\u003cbr\u003e\nA: This product includes a 1-year warranty against manufacturing defects under normal use. If something goes wrong, contact our support team with your order details. Returns are accepted within our standard return window (unused \u0026amp; in original condition); please see our policy page or reach out for instructions.\u003c\/p\u003e\n\n\u003ch2\u003eImportant Notes\u003c\/h2\u003e\n\n\u003cul\u003e\n\u003cli\u003eVerify that the product images and specifications match your original part. If discrepancies exist—even if your model is listed—do not proceed with the purchase.\u003c\/li\u003e\n\u003cli\u003eThis product is an aftermarket replacement part and is not an OEM component.\u003c\/li\u003e\n\u003cli\u003eProfessional installation is recommended to ensure safety and optimal performance.\u003c\/li\u003e\n\u003c\/ul\u003e\n\n\u003ch2\u003eWhy Choose JLM MARINE?\u003c\/h2\u003e\n\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eQuality Assured:\u003c\/strong\u003e As a trusted manufacturer for over 20 years, we supply top marine brands globally.\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eFast Global Shipping:\u003c\/strong\u003e Free delivery worldwide in 5–15 days.\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eDirect from Manufacturer:\u003c\/strong\u003e Competitive pricing without compromising quality.\u003c\/li\u003e\n\u003c\/ul\u003e","brand":"JLM Marine","offers":[{"title":"Default Title","offer_id":50579288228153,"sku":"23TH619","price":38.5,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0885\/0433\/6697\/files\/Thermostat_for_Tohatsu_Mercury_2-6_hp_4-St_3H6-01030-0_804540___345-01030-0_3H6-01030-0_-_JLM_MARINE-8291185.jpg?v=1755251566","url":"https:\/\/jlmmarine.com\/products\/18-3638-outboard-thermostat-for-mercury-mariner","provider":"JLM Marine Boat Parts Limited","version":"1.0","type":"link"}