Thermostat & Gasket Kit – Fits MerCruiser 2.5L 120HP & 3.0L 140HP (Replaces 59078T3, 59078A1, 59078Q3, 59078B, 59078)
Thermostat & Gasket Kit – Fits MerCruiser 2.5L 120HP & 3.0L 140HP (Replaces 59078T3, 59078A1, 59078Q3, 59078B, 59078) is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
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Thermostat & Gasket Kit – Fits MerCruiser 2.5L 120HP & 3.0L...
Thermostat & Gasket Kit – Fits MerCruiser 2.5L 120HP & 3.0L 140HP (Replaces 59078T3, 59078A1, 59078Q3, 59078B, 59078) is backordered and will ship as soon as it is back in stock.
Product Details
This replacement Thermostat & Gasket Kit – Fits MerCruiser 2.5L 120HP & 3.0L 140HP (Replaces 59078T3) is designed for 4-cylinder MerCruiser 120HP (2.5L) and 140HP (3.0L) inboard/sterndrive engines manufactured between 1968 and 2000. It ensures your engine maintains the correct operating temperature for optimal performance and longevity.
This kit serves as a direct replacement for Mercury/MerCruiser part numbers 59078T3, 59078A1, 59078Q3, 59078B, and 59078. It also interchanges with aftermarket parts from Sierra (18-3650), Mallory Marine (9-43153), and OBR Red Rhino (MR-524).
- Engineered for direct-fit installation, no modifications required.
- Maintains a stable engine operating temperature of 140°F (60°C).
- Complete kit includes both the thermostat and the necessary housing gasket.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercury/MerCruiser | 59078T3 |
| Mercury/MerCruiser | 59078A1 |
| Mercury/MerCruiser | 59078Q3 |
| Mercury/MerCruiser | 59078B |
| Mercury/MerCruiser | 59078 |
| Sierra | 18-3650 |
| OBR Red Rhino | MR-524 |
| Mallory Marine | 9-43153 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
This thermostat kit fits a wide range of MerCruiser engines, including specific 2.5L, 3.0L, and inline 6-cylinder models. Verify your engine's serial number falls within the specified ranges for an exact match.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| MerCruiser | 120 - 2.5L / L4 - 153 | 120 | 4-Stroke | 1968–1989 | 59078T3 | S/N 2324347 - 0C853255 |
| MerCruiser | 140 - 3.0L / L4 - 181 | 140 | 4-Stroke | 1968–1989 | 59078T3 | S/N 2508261 - 0C856558 |
| MerCruiser | 3.0L / L4 - 181 | 140 | 4-Stroke | 1990–1994 | 59078T3 | S/N 0C856559 - 0F603999 |
| MerCruiser | 3.0L / L4 - 181 | 140 | 4-Stroke | 1998–2000 | 59078T3 | S/N 0L010042 - 0L340999 |
| MerCruiser | 3.0L LX / L4 - 181 | 140 | 4-Stroke | 1990–1997 | 59078T3 | S/N 0C856559 - 0K999999 |
| MerCruiser | 165 / L6 - 250 | 165 | 4-Stroke | 1970–1971 | 59078T3 | S/N 2770032 - 5813433, Also fits inline 6 engines |
Technical Specs
- Product Type
- Thermostat Kit with Gasket
- Installation
- Direct Replacement
- Housing Material
- Metal
- Operating Temperature
- 140°F (60°C)
- Other Technical Attributes
- Fits engines with serial numbers below 0L340999
What’s in the Box
- 1 x Thermostat (Replaces 59078, 18-3550)
- 1 x Cover Gasket (Replaces 27-47590-1, 18-2831)
Installation Notes
- Ensure the engine is completely cool before beginning. Disconnect the battery.
- Drain the engine coolant into a suitable container. Locate the thermostat housing on the engine.
- Place a rag under the housing to catch any residual coolant. Remove the bolts securing the housing cover.
- Carefully lift the cover and remove the old thermostat and gasket. Note the thermostat's orientation.
- Thoroughly clean both the housing and cover mating surfaces. Remove all traces of the old gasket.
- Install the new thermostat in the correct orientation. Place the new gasket onto the housing.
- Re-install the housing cover and tighten the bolts to the manufacturer's specified torque.
- Refill the cooling system with the appropriate coolant. Reconnect the battery.
- Start the engine and check for leaks around the housing. Allow the engine to reach operating temperature, monitoring the temperature gauge.
Troubleshooting & FAQ
How do I know if my thermostat is failing?
Common signs of a faulty thermostat include engine overheating, the temperature gauge reading unusually low, or coolant leaks around the housing. The engine may also take a very long time to warm up.
Is 140°F the correct temperature for my engine?
Yes, this 140°F (60°C) thermostat is the specified temperature rating for the compatible MerCruiser 2.5L and 3.0L engines, ensuring it does not run too cold, which is crucial for engine health and efficiency.
My serial number is not listed. Will this part still fit?
This kit is confirmed to fit engines with serial numbers below 0L340999. If your serial number is higher or not listed, this part may not be compatible. We recommend checking your OEM service manual or contacting us with your engine details before purchasing.
Can I reuse my old gasket?
No. It is critical to always use a new gasket when replacing a thermostat to ensure a proper seal and prevent coolant leaks. This kit conveniently includes the correct gasket.
AINWhy is my engine still overheating after replacing the thermostat?
If overheating persists, other cooling system components may be at fault. Check for a worn impeller, clogged heat exchanger or risers, air pockets in the cooling system, or a faulty circulation pump.
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.
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