Thermostat Kit (140°F/60°C) – Fits MerCruiser 2.5L & 3.0L Stern Drives (Replaces 59078Q3, 18-3650)

Thermostat Kit (140°F/60°C) – Fits MerCruiser 2.5L & 3.0L Stern Drives

This thermostat kit is designed for MerCruiser 2.5L (120HP) and 3.0L (140HP) inline 4-cylinder and 6-cylinder GM engines (1998 and newer) equipped with cast iron exhaust manifolds. It serves as a direct replacement component for maintaining optimal engine operating temperature.

Engineered to match OEM specifications, this kit directly replaces MerCruiser part numbers 59078Q3, 59078T3, 59078T4, and others, along with equivalent parts from Sierra, GLM, and Mallory.

• Regulates engine temperature to a consistent 140°F (60°C).
• Designed for raw water-cooled engine applications.
• Kit includes both the thermostat and replacement cover gasket for a complete installation.

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 x Thermostat (59078)
• 1 x Thermostat Cover Gasket (47590)

Installation Guide

1. Drain the Coolant: Ensure the engine is cool. Place a container under the drain plug and empty the cooling system.
2. Locate Housing: Find the thermostat housing, typically on the top front of the engine where the main coolant hose connects.
3. Remove Housing: Loosen and remove the bolts securing the housing cover. Gently tap it if it is stuck, but do not pry.
4. Remove Old Parts: Take out the old thermostat and peel off the old gasket. Note the thermostat's orientation.
5. Clean Surfaces: Thoroughly clean the mating surfaces of the housing and engine block to ensure a proper seal.
6. Install New Thermostat: Place the new thermostat into the housing, ensuring it is in the correct position.
7. Install New Gasket: Position the new gasket on the housing.
8. Re-install Housing: Secure the housing cover and tighten the bolts to the manufacturer's recommended torque.
9. Refill and Check: Refill the cooling system. Start the engine, let it warm up, and carefully check for any leaks around the housing. Monitor the temperature gauge to ensure it stays in the normal range.

Troubleshooting & FAQ

Q: How do I confirm this kit fits my MerCruiser engine?
A: Start by checking the Compatible Engine Models table on this page. Verify your engine model, horsepower, and any serial number ranges listed. Cross-reference your existing part number with our Part Number Cross-Reference table to ensure it is a direct replacement.

Q: What are the signs of a failing thermostat?
A: Common signs include engine overheating (staying hot) or running too cold (not reaching operating temperature). You may also notice erratic temperature gauge readings or coolant leaks around the thermostat housing.

Q: How often should I replace my engine's thermostat?
A: While there is no fixed interval, it is good practice to inspect the thermostat during routine cooling system service (every 2-3 years) or replace it preventatively, especially in saltwater environments where corrosion is more likely.

Q: OEM vs. 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.