18-3642 Outboard Thermostat for Mercury Mariner 40HP 3Cyl | JLM Marine

18-3642 Outboard Thermostat for Mercury Mariner 40HP 3Cyl

This thermostat is engineered to maintain optimal engine operating temperatures for Mercury Mariner 40HP 3-cylinder outboard motors (serial number 1C049739 and up). Designed as a direct replacement for multiple OEM part numbers, this component ensures reliable cooling system performance and prevents overheating damage.

This aftermarket thermostat replaces OEM numbers 825212, 825212 A1, 825212001, 855676003, 825212T02, and 855676A1, providing broad compatibility across Mercury Mariner outboard applications.

• Complete installation kit with thermostat and rubber seal ring
• Fits Mercury Mariner 40HP 3-cylinder outboards from serial 1C049739 onward
• Direct replacement for multiple OEM part numbers

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. Disconnect the battery and allow the engine to cool completely before beginning work.
2. Drain the cooling system by removing the lower drain plug and allowing coolant to empty into a container.
3. Locate the thermostat housing on the cylinder head and remove the retaining bolts using the appropriate socket.
4. Carefully remove the old thermostat housing cover and inspect for corrosion or damage.
5. Remove the old thermostat and rubber seal ring, cleaning all mating surfaces thoroughly with a gasket scraper.
6. Install the new rubber seal ring into the groove on the thermostat or housing (follow OEM positioning).
7. Position the new thermostat into the housing with the correct orientation (spring side typically faces down/inward).
8. Reinstall the thermostat housing cover and tighten bolts to manufacturer specifications in a cross-pattern.
9. Reinstall the lower drain plug and refill the cooling system with fresh coolant to the proper level.
10. Reconnect the battery and start the engine, checking for leaks around the thermostat housing.
11. Allow the engine to reach operating temperature and verify proper cooling system function.
12. Recheck coolant level after initial run and top off if necessary.

Troubleshooting & FAQ

Q: How do I confirm this thermostat fits my Mercury Mariner outboard?
A: Verify that your engine is a 40HP 3-cylinder model with serial number 1C049739 or higher. Cross-reference your current thermostat part number with the OEM numbers listed (825212, 825212 A1, 825212001, 855676003, 825212T02, or 855676A1). You can also check Mercury's parts diagrams using your serial number to confirm compatibility.

Q: How often should I replace the thermostat on my outboard?
A: Replace the thermostat if you experience overheating, inconsistent operating temperatures, or poor engine performance. As preventive maintenance, inspect the thermostat annually and replace every 3–5 years or per your engine manufacturer's service schedule, especially in saltwater environments.

Q: What are the signs that my thermostat is failing?
A: Common symptoms include engine overheating, running too cool (poor fuel efficiency), erratic temperature gauge readings, or coolant leaks around the thermostat housing. If the engine takes longer than normal to warm up or frequently triggers overheat alarms, the thermostat may be stuck open or closed.

Q: Should I replace the rubber seal ring every time I replace the thermostat?
A: Yes, always replace the rubber seal ring when installing a new thermostat to ensure a proper seal and prevent coolant leaks. This package includes a new seal ring for complete installation.

Q: Is this an OEM thermostat or an aftermarket replacement?
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.