Thermostat Repair Service Kit for Mercruiser 2.5L 3.0L 120 140 165 GM SternDrive Motors 59078T3

Thermostat Kit (140°F) – Fits MerCruiser 120/140/165 HP 2.5L/3.0L Engines (Replaces 59078T3)

This thermostat kit is engineered for MerCruiser 2.5L and 3.0L 4-cylinder engines (120-140 HP) and 4.1L 6-cylinder engines (165 HP) manufactured between 1964 and 1997. It ensures your engine maintains the correct operating temperature for optimal performance and longevity.

This kit serves as a direct replacement for a wide range of OEM and aftermarket parts, providing a reliable solution for maintenance and repair.

• Operating Temperature: Maintains a consistent 140°F (60°C).
• Installation Method: Designed for direct replacement of the original thermostat and gasket.
• Core Components: Includes one thermostat and one thermostat cover gasket.

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

Installation Guide

1. Safety First: Ensure the engine is cool and disconnect the battery.
2. Drain Coolant: Drain the engine block coolant into a suitable container.
3. Locate Housing: Find the thermostat housing, typically where the main coolant hose from the circulator pump connects to the engine.
4. Remove Housing: Loosen and remove the bolts securing the housing cover.
5. Extract Old Parts: Remove the old thermostat and gasket. Note the thermostat's orientation.
6. Clean Surfaces: Thoroughly clean the mating surfaces of the housing and engine block, removing all old gasket material.
7. Install New Parts: Place the new thermostat into the block in the correct orientation, followed by the new gasket.
8. Reassemble: Reinstall the housing cover and tighten the bolts to the manufacturer's recommended torque.
9. Refill and Check: Refill the cooling system, reconnect the battery, and start the engine. Check for any leaks around the housing and monitor the temperature gauge.

Troubleshooting & FAQ

How do I confirm this kit fits my MerCruiser engine?
Verify your engine's model, horsepower, and year of manufacture against our "Compatible Engine Models" table. For best results, also compare your existing part number to the numbers listed in the "Part Number Cross-Reference" section.

What are common signs of a failing thermostat?
Symptoms include engine overheating, the engine not reaching its proper operating temperature (running too cool), erratic readings on the temperature gauge, or coolant leaks originating from the thermostat housing area.

What's the difference between OEM part numbers like 59078T3, 59078Q3, and 59078T4?
These different suffixes typically indicate minor revisions, material changes, or packaging updates made by the manufacturer over the years. This aftermarket kit is designed to be a functional replacement for all listed variants, providing the same fit and 140°F temperature specification.

OEM vs. Aftermarket?
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.

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

What is your warranty and return policy?
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.