18-3638 Outboard Thermostat for Mercury Mariner | JLM Marine

Thermostat for Mercury Mariner Outboard – Fits 2.5-6 HP Models

This 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.

The 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 & Up) and 4-6 HP models (serial 0R411838 & Up).

• Complete kit includes thermostat and rubber seal ring
• Direct replacement for OEM 804540 and 853704
• Ensures proper engine temperature control

Part Number Cross-Reference

Use this table to confirm equivalent part numbers across OEM and aftermarket brands.

Compatible Engine Models

Confirm your engine model/year in the table below. For Mercury owners, here's a guide to help you identify your engine's serial/model: Mercury Serial Number Guide. You can also review exploded diagrams to verify part numbers: Mercury Parts Diagrams & Catalog.

Technical Specifications

Package Contents

• 1 × Thermostat
• 1 × Ring Rubber Seal

Installation Guide

1. Allow the engine to cool completely before beginning work. Ensure the ignition is off and disconnect the battery if applicable.
2. Locate the thermostat housing on your Mercury Mariner outboard engine. It is typically found near the cylinder head or water jacket outlet.
3. Remove the mounting bolts securing the thermostat housing cover using the appropriate wrench or socket. Keep track of all fasteners.
4. Carefully lift the housing cover and remove the old thermostat and seal. Inspect the housing for corrosion or debris and clean thoroughly.
5. Install the new rubber seal ring into the groove of the thermostat housing. Ensure it seats properly without twisting or pinching.
6. Position the new thermostat in the housing with the temperature-sensing element facing the correct direction (typically toward the engine block).
7. Replace the housing cover and hand-tighten the mounting bolts. Then torque bolts to manufacturer specifications in a crisscross pattern to ensure even sealing.
8. Reconnect the battery if disconnected. Start the engine and allow it to reach operating temperature while monitoring for leaks around the thermostat housing.
9. Check coolant levels and top off if necessary. Verify that the engine temperature remains within normal operating range.

Troubleshooting & FAQ

Q: How do I confirm this thermostat fits my Mercury Mariner outboard?
A: Check your engine's serial number plate. This thermostat fits 2.5-3.5 HP models with serial 0R131842 & up, and 4-6 HP models with serial 0R411838 & up. Cross-reference your current part number with OEM 804540 or 853704.

Q: What are the signs that my thermostat needs replacement?
A: 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.

Q: How often should I replace the thermostat on my outboard?
A: 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.

Q: Does this kit include the gasket or seal?
A: Yes, this package includes both the thermostat and the rubber seal ring required for proper installation. No additional seals need to be purchased separately.

Q: Is this an OEM part or aftermarket?
A: 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.

Q: How fast do you ship and how much does it cost?
A: 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.

Q: What is your warranty and return policy?
A: 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 & in original condition); please see our policy page or reach out for instructions.

Important Notes

• Verify 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.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.

Why Choose JLM MARINE?

• Quality Assured: As a trusted manufacturer for over 20 years, we supply top marine brands globally.
• Fast Global Shipping: Free delivery worldwide in 5–15 days.
• Direct from Manufacturer: Competitive pricing without compromising quality.