Thermostat Repair Kit (140°F) – Fits Mercruiser 2.5L/3.0L 120/140/165 HP (Replaces 59078Q3, 59078T3, 59078T4)
Thermostat Repair Kit (140°F) – Fits Mercruiser 2.5L/3.0L 120/140/165 HP (Replaces 59078Q3, 59078T3, 59078T4) is backordered and will ship as soon as it is back in stock.
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Worry-free returns
Worry-free returns
You may return most new, unopened items within 7 days of delivery for a full refund. We'll also pay the return shipping costs if the return is a result of our error (you received an incorrect or defective item, etc.).
You should expect to receive your refund within four weeks of giving your package to the return shipper, however, in many cases you will receive a refund more quickly. This time period includes the transit time for us to receive your return from the shipper (5 to 10 business days), the time it takes us to process your return once we receive it (3 to 5 business days), and the time it takes your bank to process our refund request (5 to 10 business days).
If you need to return an item, simply login to your account, view the order using the "Complete Orders" link under the My Account menu and click the Return Item(s) button. We'll notify you via e-mail of your refund once we've received and processed the returned item.
Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Thermostat Repair Kit (140°F) – Fits Mercruiser 2.5L/3.0L 120/140/1...
Product Details
This Thermostat Repair Kit (140°F) – Fits Mercruiser 2.5L/3.0L 120/140/165 HP Engines is engineered to restore proper engine operating temperature in a range of GM-based MerCruiser powerplants. It's designed for 4-cylinder (2.5L/3.0L) and 6-cylinder inline engines manufactured between 1964 and 1997, ensuring reliable performance in raw or freshwater cooling systems.
This kit serves as a direct replacement for Mercruiser part numbers 59078Q3, 59078T3, and 59078T4, as well as aftermarket equivalents from Sierra, GLM, and Mallory. It includes the essential components for a complete thermostat service.
- Installs as a direct-fit replacement for the original thermostat and gasket.
- Maintains a consistent 140°F (60°C) operating temperature.
- Includes a brass thermostat and a precision-cut cover gasket.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercruiser | 59078Q3 |
| Mercruiser | 59078T3 |
| Mercruiser | 59078T4 |
| Mercruiser | 59078 |
| Mercruiser | 59078A1 |
| Mercruiser | 59078B |
| Sierra | 18-3650 |
| Sierra | 18-3550 |
| Sierra | 18-2831 |
| GLM | 13020 |
| GLM | 13370 |
| GLM | 31430 |
| Mallory | 9-43153 |
| Mallory | 9-43020 |
| Mallory | 9-60012 |
| Barr | MC-29-59078 |
| OMC | 0508543 |
| OMC | 0508617 |
| Force | F84068 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
This kit is verified for the following MerCruiser models. Compatibility may extend to other inline 4 and 6-cylinder GM engines not listed here.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| MerCruiser | 120 GM | 120 | 4-Stroke | 1964–1986 | 59078T3 | Inline 4-cylinder |
| MerCruiser | 140 GM | 140 | 4-Stroke | 1968–1986 | 59078T3 | Inline 4-cylinder |
| MerCruiser | 165 GM | 165 | 4-Stroke | 1970–1981 | 59078T3 | Inline 6-cylinder |
| MerCruiser | 2.5L GM | 120 | 4-Stroke | 1987–1989 | 59078T3 | Inline 4-cylinder |
| MerCruiser | 3.0L GM | 140 | 4-Stroke | 1990–1997 | 59078T3 | Inline 4-cylinder |
| MerCruiser | 3.0LX GM | 140 | 4-Stroke | 1990–1997 | 59078T3 | Inline 4-cylinder |
Technical Specs
- Product Type
- Thermostat Kit
- Installation
- Direct replacement
- Temperature Rating
- 140°F (60°C)
- Operating Temperature
- 140°F (60°C)
- Housing Material
- Brass
- Contents
- Thermostat and cover gasket
- Warranty
- 1 Year (varies by retailer)
What’s in the Box
- 1 x Thermostat (Replaces 59078T3)
- 1 x Thermostat Cover Gasket (Replaces 47590)
Installation Notes
- Ensure the engine is completely cool before beginning.
- Locate the thermostat housing on the engine and place a drain pan underneath.
- Drain the engine coolant to a level below the thermostat housing.
- Remove the bolts securing the housing cover. Note the orientation of the housing.
- Carefully remove the old thermostat and gasket. Clean any old gasket material from the housing and engine surfaces.
- Install the new thermostat, ensuring it is correctly seated in the direction of coolant flow.
- Place the new gasket onto the housing, aligning it with the bolt holes.
- Reinstall the housing cover and tighten the bolts to the manufacturer's specified torque.
- Refill the cooling system with the appropriate coolant.
- Start the engine and check for any leaks around the housing. Allow the engine to reach operating temperature to confirm the thermostat opens correctly.
Troubleshooting & FAQ
How do I know if my thermostat is failing?
Common signs of a faulty thermostat include engine overheating (stuck closed) or taking too long to reach operating temperature (stuck open). You may also notice erratic temperature gauge readings or poor engine performance.
Is this thermostat compatible with inline 6-cylinder engines other than the 165 GM?
While this kit is only officially documented for the MerCruiser 165 model, some sources indicate it may fit other similar GM-based inline 6-cylinder marine engines. However, you must verify the part number (59078 series) and temperature rating (140°F) against your engine's service manual before purchasing.
Why is a 140°F thermostat used in marine engines?
A 140°F thermostat is standard for raw-water cooled marine engines because it keeps the engine warm enough for efficient combustion but cool enough to prevent salt and mineral deposits from building up in the cooling passages, which can occur at higher temperatures.
How often should a marine thermostat be replaced?
It is good practice to inspect your thermostat annually and replace it every 2-3 years as part of preventative maintenance, or immediately if you notice any signs of malfunction. For boats used in saltwater, annual replacement is often recommended.
What's the difference between OEM and aftermarket parts?
This listing is an aftermarket replacement (not OEM). It's built to OEM-level specifications with comparable materials and performance. Our parts are manufactured in an ISO 9002 facility and are batch-inspected for quality and consistency.
What are your shipping speed and cost?
We ship worldwide quickly—typical transit is 5–15 days. Standard shipping is free for most orders, and expedited options are available at checkout where applicable.
What is the warranty and return policy?
This product is covered by a 1-year warranty against manufacturing defects under normal marine use. If you encounter an issue, please contact our support team with your order details. Returns are accepted within our standard window for unused items in their original condition. See our policy page or reach out for instructions.
Safety & Important Info
- Verify images and specifications against your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement, not an OEM component.
- Professional installation is recommended for safety and performance.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
- Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
- Manufacturer-Direct Value — Fair pricing without compromising on materials or quality control.
Payment & Security
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