Thermostat Kit (140°F/60°C) – Fits MerCruiser 2.5L & 3.0L (Replaces 59078Q3, 18-3650)
Thermostat Kit (140°F/60°C) – Fits MerCruiser 2.5L & 3.0L (Replaces 59078Q3, 18-3650) 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 Kit (140°F/60°C) – Fits MerCruiser 2.5L & 3.0L (Repl...
Product Details
This Thermostat Kit (140°F/60°C) is engineered for MerCruiser 2.5L (120HP) and 3.0L (140HP) inline 4-cylinder and 6-cylinder GM engines from 1998 and newer equipped with cast iron exhaust manifolds. 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 part numbers from brands including MerCruiser, Sierra, Force, GLM, Mallory, and Barr.
- Installation: Designed as a direct OEM-equivalent replacement.
- Application: Specifically for raw water-cooled marine engines.
- Complete Kit: Includes both the thermostat and housing gasket for a full service.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| MerCruiser | 59078, 59078A1, 59078B, 59078Q3, 59078T3, 59078T4 |
| Sierra | 18-3650, 18-3550, 18-2831 |
| Force | F84068, F84068-1, F84068-2, F85068-2 |
| GLM | 13370, 13020, 31430 |
| Mallory | 9-43153, 9-43020, 9-60012 |
| Barr | MC472747590 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
Compatibility for MerCruiser models is detailed below. Verify your engine's serial number and specifications to ensure a correct match.
| Brand | Model | Rated HP | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|
| MerCruiser | MCM GM IN-LINE 4 & 6 Cyl engines | - | 1998–Up | 59078Q3, 59078T3, 59078T4 | - |
| MerCruiser | 120 / 2.5L GM | 120 | - | 59078, 59078A1, 59078B | For s/n 2324347 & Up w/cast iron exhaust manifolds |
| MerCruiser | 140 / 3.0L GM | 140 | - | 59078, 59078A1, 59078B | For s/n 0L340999 & below |
| MerCruiser | 3.0L / LX GM | 3.0L | - | 59078, 59078A1, 59078B | - |
| MerCruiser | 165 GM | 165 | - | 59078, 59078A1, 59078B | - |
Technical Specs
- Product Type
- Marine Engine Thermostat
- Installation
- Direct OEM replacement
- Temperature Rating
- 140°F (60°C)
- Application
- Raw water-cooled engines
- Warranty
- 1 year limited manufacturer's warranty against defects
- Other Technical Attributes
- For cooling system temperature regulation
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.
- Drain the engine coolant to a level below the thermostat housing.
- Carefully remove the bolts from the thermostat housing cover.
- Lift the cover and remove the old thermostat and gasket.
- Thoroughly clean all mating surfaces on the housing and cover, removing any old gasket residue.
- Install the new thermostat, making sure it is oriented correctly as per your engine's service manual.
- Position the new gasket on the housing.
- Re-install the cover and tighten the bolts evenly to the manufacturer's specified torque.
- Refill the cooling system with the appropriate coolant.
- Start the engine and carefully check for any leaks around the housing as it warms up.
- Allow the engine to reach its full operating temperature to verify the new thermostat opens correctly.
Troubleshooting & FAQ
How do I know if this thermostat fits my engine?
This kit is designed for MerCruiser 2.5L and 3.0L inline 4/6-cylinder GM engines from 1998 and newer with cast iron exhaust manifolds. Please cross-reference your part number using our Part Number Matrix and check the detailed compatibility table to confirm fitment for your specific model and serial number range.
What are the signs of a failing thermostat?
Common symptoms include engine overheating (if the thermostat is stuck closed) or the engine failing to reach normal operating temperature (if it's stuck open). An engine that runs too cool can suffer from poor performance, incomplete fuel combustion, and carbon buildup.
How often should a marine thermostat be replaced?
As a preventative measure, it is good practice to inspect the thermostat during routine cooling system service and consider replacement every 3-5 years, or immediately if you notice temperature regulation issues. Engines operated in saltwater may require more frequent inspections and changes.
Why is a 140°F thermostat used?
A 140°F (60°C) thermostat is standard for many raw water (or open) cooling systems. It helps the engine warm up to an efficient operating temperature without getting hot enough to cause salt and minerals to precipitate and clog cooling passages.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.
Shipping speed & cost?
We ship worldwide quickly—typical transit 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
Warranty & returns?
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 (unused & in original condition). See our policy page or reach out for instructions.
Safety & Important Info
- Verify images/specifications vs 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 QC.
Payment & Security
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