140°F (60°C) Thermostat Kit for MerCruiser 2.5L & 3.0L Inline 4/6-Cylinder Engines – OEM 59078Q3, Sierra 18-3650
140°F (60°C) Thermostat Kit for MerCruiser 2.5L & 3.0L Inline 4/6-Cylinder Engines – OEM 59078Q3, Sierra 18-3650 is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
Delivery and Shipping
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140°F (60°C) Thermostat Kit for MerCruiser 2.5L & 3.0L Inline 4...
140°F (60°C) Thermostat Kit for MerCruiser 2.5L & 3.0L Inline 4/6-Cylinder Engines – OEM 59078Q3, Sierra 18-3650 is backordered and will ship as soon as it is back in stock.
Product Details
This 140°F (60°C) Thermostat Kit is intended for selected MerCruiser 2.5L and 3.0L GM inline 4- and 6-cylinder engines. It helps regulate engine operating temperature in the cooling system and is supplied as a service kit containing the thermostat and the required thermostat gasket.
The thermostat is identified by MerCruiser OEM part number 59078Q3. The reference is also associated with aftermarket replacement numbers including Sierra 18-3650. Always compare the engine model, year, serial number, cooling arrangement, and original thermostat before ordering.
- Temperature rating: 140°F (60°C)
- Application: Selected MerCruiser GM inline 4- and 6-cylinder engines
- Service type: Thermostat and gasket replacement kit
- Installation: Direct replacement when the listed engine and serial-number conditions match
Part Number Matrix
Use these references as cross-reference leads, then confirm the complete engine application before purchase.
| Brand or Reference | Part Number |
|---|---|
| MerCruiser OEM | 59078Q3 |
| Sierra | 18-3650 |
| GLM | 13370 |
| Related MerCruiser references | 59078, 59078A1, 59078B, 59078T3, 59078T4 |
Engine & Model Compatibility
The following applications are based on cataloged MerCruiser GM inline-engine applications for the 140°F thermostat family. Model, year, and serial-number conditions must be considered together.
| Brand | Model | Rated HP / Displacement | Years | Serial Number Condition | Notes |
|---|---|---|---|---|---|
| MerCruiser | 120 / 2.5L GM | 120 HP / 2.5L | 1972–1989 | 3377341–0C853255 | Inline GM application; verify the original thermostat and housing configuration. |
| MerCruiser | 140 / 3.0L GM | 140 HP / 3.0L | 1968–1989 | 2508261–0C856558 | Inline GM application; verify the original thermostat and housing configuration. |
| MerCruiser | 3.0L / LX GM | 3.0L | 1990–1998 | 0C856559 and up | Confirm the serial-number range and thermostat arrangement before ordering. |
Technical Specs
- Product Type
- Marine engine thermostat kit
- Thermostat Rating
- 140°F (60°C)
- Engine Configuration
- Selected MerCruiser GM inline 4- and 6-cylinder engines
- Primary OEM Reference
- MerCruiser 59078Q3
- Kit Contents
- Thermostat and thermostat gasket
- Application Type
- Cooling-system temperature regulation
- Installation
- Direct replacement when fitment is confirmed
What’s in the Box
- 1 × 140°F (60°C) thermostat
- 1 × thermostat gasket, reference 24-47590-1
Installation Notes
- Allow the engine to cool completely before beginning service.
- Close the seacock or otherwise isolate the water supply as required by the boat and engine configuration.
- Drain cooling water or coolant to a level below the thermostat housing, as applicable.
- Remove the thermostat-housing cover carefully and retain the fasteners.
- Remove the old thermostat and gasket.
- Clean the housing and cover mating surfaces without damaging them.
- Install the new thermostat in the same orientation as the original component and the engine service manual specifies.
- Position the new gasket correctly between the mating surfaces.
- Reinstall the cover and tighten the fasteners evenly to the manufacturer’s specified torque.
- Restore the cooling-water or coolant level as applicable.
- Run the engine and inspect the housing for leaks.
- Confirm that the engine reaches and maintains the expected operating temperature.
Troubleshooting & FAQ
How do I know if this thermostat fits my engine?
Confirm that your engine is a compatible MerCruiser GM inline 4- or 6-cylinder model, then match the year and serial-number range in the compatibility table. Also compare the original thermostat, housing, gasket arrangement, and temperature rating before ordering.
What is the thermostat temperature rating?
This kit uses a 140°F (60°C) thermostat. Do not substitute it for a different temperature rating unless the engine manufacturer’s specifications call for that rating.
What are common signs of a failing thermostat?
Possible symptoms include overheating, slow warm-up, operation that is consistently too cool, unstable temperature readings, or poor temperature regulation. These symptoms can also be caused by restricted passages, water-pump problems, sensors, or other cooling-system faults.
Does the kit include the gasket?
Yes. The kit includes one thermostat and one thermostat gasket identified in the catalog reference as 24-47590-1.
Is this an OEM MerCruiser part?
This listing is for an aftermarket replacement kit identified for the MerCruiser OEM reference 59078Q3. It is not represented as a genuine MerCruiser-branded component.
Can I use this kit on every 2.5L or 3.0L MerCruiser engine?
No. Displacement alone is not sufficient to confirm fitment. Model, production year, serial number, thermostat housing, cooling arrangement, and original part number must all be checked together.
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?
Returns are accepted within our standard window when the item is unused and in its original condition. Contact support with your order details if you need return instructions or assistance with an order.
Safety & Important Info
- Verify the engine model, year, serial number, original part number, gasket arrangement, and thermostat temperature before purchase.
- Do not rely on displacement or horsepower alone to determine compatibility.
- Allow the engine to cool before opening the cooling system.
- Follow the engine manufacturer’s service manual for torque values, sealing procedures, and cooling-system bleeding or filling requirements.
- Professional installation is recommended when the cooling-system configuration or fitment is uncertain.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts selected for dependable service in real-world operating conditions.
- Fast Global Fulfillment — Free standard delivery is available to most regions, with typical transit of 5–15 days.
- Manufacturer-Direct Value — Fair pricing with careful product selection and quality-control focus.
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