Thermostat Repair Kit (140°F) – Fits Mercruiser 2.5L/3.0L 120/140/165 HP Engines (Replaces 59078Q3, 59078T3, 59078T4)

Thermostat Repair Kit (140°F) – Fits Mercruiser 2.5L/3.0L Engines

This thermostat repair kit is designed for Mercruiser 120-165 HP, 4-stroke GM inline 4-cylinder (2.5L/3.0L) and 6-cylinder engines from 1964–1997. It ensures your engine maintains the correct operating temperature for optimal performance and longevity.

Engineered as a direct replacement for multiple OEM and aftermarket part numbers, this kit simplifies maintenance and guarantees a precise fit. It contains the essential components for a complete thermostat replacement.

• Operating Temperature: Maintains a consistent 140°F (60°C).
• Included Parts: Comes with one thermostat and one cover gasket for a complete installation.
• Installation: Designed for direct, hassle-free replacement of the original unit.

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.

Technical Specifications

Package Contents

• 1 x Thermostat (Replaces 59078T3)
• 1 x Thermostat Cover Gasket (Replaces 47590)

Installation Guide

1. Safety First: Ensure the engine is completely cool and disconnect the battery.
2. Drain Coolant: Place a container under the engine and drain the cooling system to a level below the thermostat housing.
3. Locate Housing: Find the thermostat housing, typically at the top front of the engine where the main coolant hose connects.
4. Remove Hoses: Loosen the hose clamps and carefully detach the coolant hoses from the housing.
5. Unbolt Housing: Remove the bolts securing the thermostat housing cover. Note their placement for reassembly.
6. Remove Old Thermostat: Lift the cover off and remove the old thermostat and gasket. Take note of the thermostat's orientation.
7. Clean Surfaces: Thoroughly clean the mating surfaces of the housing and cover, removing all old gasket material.
8. Install New Gasket & Thermostat: Place the new gasket and thermostat into the housing, ensuring the thermostat is correctly oriented.
9. Re-secure Housing: Re-attach the housing cover and tighten the bolts evenly in a star pattern to the manufacturer's specified torque.
10. Reconnect Hoses: Re-attach the coolant hoses and secure the clamps.
11. Refill Coolant: Refill the cooling system with the appropriate coolant mixture.
12. Check for Leaks: Reconnect the battery, start the engine, and let it warm up. Check carefully for any leaks around the housing.

Troubleshooting & FAQ

Q: How can I confirm this thermostat fits my specific engine?
A: This kit is designed for MerCruiser 2.5L and 3.0L inline 4-cylinder engines, and some inline 6-cylinder models, from 1964–1997. The most reliable method is to cross-reference your existing part number with the numbers listed in our table, including 59078Q3, 59078T3, and Sierra 18-3650.

Q: What are the signs of a failing thermostat?
A: Common signs include engine overheating, the temperature gauge reading unusually low, or significant and erratic temperature fluctuations. A thermostat stuck open will prevent the engine from reaching its proper operating temperature, while one stuck closed will cause it to overheat.

Q: Why is a 140°F thermostat used?
A: A 140°F (60°C) thermostat is specified for raw water cooled marine engines to prevent salt and mineral buildup that can occur at higher temperatures. This helps protect internal engine passages from corrosion and blockages.

Q: Is this an OEM part?
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.