Thermostat Kit (140°F/60°C) – Fits MerCruiser 2.5L/3.0L GM 4 & 6 Cylinder (Replaces 59078Q3, 59078T3)
Thermostat Kit (140°F/60°C) – Fits MerCruiser 2.5L/3.0L GM 4 & 6 Cylinder (Replaces 59078Q3, 59078T3) is backordered and will ship as soon as it is back in stock.
Couldn't load pickup availability
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 Kit (140°F/60°C) – Fits MerCruiser 2.5L/3.0L GM 4 & ...
Product Details
This is an aftermarket Thermostat Kit (140°F/60°C) – Fits MerCruiser 2.5L/3.0L GM 4 & 6 Cylinder designed for MerCruiser 120-165 HP inline 4 & 6 cylinder engines from 1964–1998. It is specifically compatible with 2.5L (120 HP) and 3.0L (140 HP) GM sterndrive applications, ensuring reliable temperature regulation.
This kit serves as a direct replacement for multiple part numbers, including Mercury/MerCruiser 59078Q3, Sierra 18-3650, GLM 13370, Mallory 9-43153, and Force F84068 series references.
- Installs as a direct OEM-equivalent replacement.
- Engineered for use in closed cooling systems.
- The kit includes one thermostat and one thermostat cover gasket for a complete installation.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercury/MerCruiser | 59078Q3 |
| Mercury/MerCruiser | 59078T3 |
| Mercury/MerCruiser | 59078T4 |
| Mercury/MerCruiser | 59078 |
| Mercury/MerCruiser | 59078A1 |
| Mercury/MerCruiser | 59078B |
| Mercury/MerCruiser | 47590 |
| Sierra | 18-3650 |
| Sierra | 18-3550 |
| Sierra | 18-2831 |
| GLM | 13370 |
| GLM | 13020 |
| GLM | 31430 |
| Mallory | 9-43153 |
| Mallory | 9-43020 |
| Mallory | 9-60012 |
| Force | F84068 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
The following MerCruiser Stern Drive models are compatible with this thermostat kit.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| MerCruiser | 2.5L GM 153 I/L4 | 120 | 4-Stroke | 1964–1989 | 59078Q3 | Also known as 120 GM 153 I/L4 |
| MerCruiser | 3.0L GM 181 I/L4 | 140 | 4-Stroke | 1968–1998 | 59078Q3 | Also known as 140 GM 181 I/L4 |
| MerCruiser | 3.0LX GM | 140 | 4-Stroke | 1990–1997 | 59078Q3 | |
| MerCruiser | 165 GM 250 I/L6 | 165 | 4-Stroke | 1970–1981 | 59078Q3 |
Technical Specs
- Product Type
- Marine Engine Thermostat Kit
- Installation
- Direct OEM replacement
- Temperature Rating
- 140°F (60°C)
- Operating Temperature
- 140°F (60°C) opening temperature
- Application
- Closed cooling system
- Housing Material
- Metal (thermostat) with rubber gasket
- Product Dimensions
- 7.0" x 11.0" x 11.0"
- Weight
- 3.2 ounces (90.7 g)
- Other Technical Attributes
- Direct drop-in replacement for OEM components
What’s in the Box
- 1 x Thermostat (59078)
- 1 x Thermostat Cover Gasket (47590)
Installation Notes
- Ensure the engine is completely cool before starting. Disconnect the battery for safety.
- Drain the engine coolant from the system into a suitable container.
- Locate the thermostat housing on the engine. It is typically found where the upper radiator hose connects to the engine block.
- Loosen and remove the bolts securing the thermostat housing cover.
- Carefully remove the old thermostat and gasket. Note the orientation of the existing thermostat for correct placement of the new one.
- Thoroughly clean both the thermostat housing and the cover mating surfaces to remove any old gasket material or sealant.
- Install the new thermostat, ensuring it is seated correctly in the housing.
- Place the new gasket into position on the housing.
- Reinstall the thermostat housing cover and tighten the bolts to the manufacturer's recommended torque specifications.
- Refill the cooling system with the appropriate type and amount of coolant.
- Reconnect the battery. Start the engine, let it warm up, and carefully check for any coolant leaks around the thermostat housing.
- Top off the coolant level if necessary after the engine has cooled down.
Troubleshooting & FAQ
How do I know if my thermostat is failing?
Common signs of a failing thermostat include engine overheating, the engine running too cold (gauge not reaching normal operating temperature), erratic temperature fluctuations, and coolant leaks from the thermostat housing area.
What is the correct opening temperature for this kit?
This thermostat is rated to begin opening at 140°F (60°C), which is the standard specification for the listed MerCruiser engine applications to ensure optimal performance and efficiency.
Does the installation orientation matter?
Yes, orientation is critical. The thermostat must be installed in the correct position (typically with the spring-side facing the engine) to regulate coolant flow correctly. Installing it backward will cause overheating. Consult your service manual for confirmation.
How often should a marine thermostat be replaced?
It is recommended to inspect your thermostat during regular cooling system maintenance (every 1-2 years for raw water cooled, 3-4 for fresh water). Replace it proactively or as soon as you notice any signs of temperature regulation issues to prevent engine damage.
Is this a complete kit for the job?
This kit includes the thermostat and the necessary housing gasket, which are the core components for replacement. You may require additional sealant or coolant depending on your specific service procedures.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). It is built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility and batch-inspected for consistency.
Shipping speed & cost?
We ship worldwide quickly—typical transit is 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
Warranty & returns?
This part is covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window if unused and in original condition. See our policy page or reach out for instructions.
Safety & Important Info
- Verify images and specifications against your original part before purchase. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement component, not an OEM part.
- Professional installation is recommended for safety and optimal 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
Payment methods
Your payment information is processed securely. We do not store credit card details nor have access to your credit card information.
