Mercury Quicksilver Thermostat 8M0139434 — 125°F | 15 & 20 HP EFI FourStroke
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Mercury Quicksilver Thermostat — OEM 8M0139434 | 125°F | 15 & 20 HP EFI FourStroke
This genuine OEM thermostat regulates cooling-water flow on selected Mercury 15 HP and 20 HP EFI FourStroke outboards, helping the engine reach and hold the correct operating temperature for efficient combustion and long-term reliability. The 8M0139434 uses a self-cleaning design that allows cooling-system particles to pass through the closing surfaces rather than accumulate — an important feature in saltwater and silty environments. Rated at 125°F, it is specific to the 2-cylinder EFI configuration and serial range 0R833820 and above.
Part Information
- OEM Part Number: 8M0139434
- Brand: Mercury / Quicksilver
- Official Product Type: Engine cooling-system thermostat
- Temperature Rating: 125°F
- Engine Configuration: 2-cylinder EFI FourStroke
- Serial Range: 0R833820 and above
- Design: Self-cleaning thermostat
- System: Engine cooling system
- GTIN / UPC: 745061050396
Compatible Applications
- Mercury 15 HP EFI FourStroke — 2-cylinder, serial 0R833820 and above (2017 and newer)
- Mercury 20 HP EFI FourStroke — 2-cylinder, serial 0R833820 and above (2017 and newer)
- Confirm engine horsepower, cylinder count, and serial number before ordering — earlier 15/20 HP FourStroke engines below serial 0R833820 may use a different thermostat
Related Parts
- Mercury Outboard Thermostat Assembly 120°F — 8M0090819
- Mercury Outboard Thermostat 140°F — 8M0119207
When to Replace
- Engine runs consistently below normal operating temperature (thermostat stuck open)
- Engine overheats and cooling-system diagnosis points to a faulty thermostat (stuck closed)
- Thermostat fails to open or close correctly on bench testing
- As part of a scheduled cooling-system service or when the thermostat housing is opened for any reason
Service Interval
Inspect the thermostat annually or whenever the cooling system is serviced. Replace immediately if the thermostat fails to open or close at the correct temperature on testing. Many technicians recommend replacing the thermostat any time the thermostat housing is removed, as reinstalling a used thermostat is a leading cause of repeat cooling-system issues.
Installation Notes
- Allow the engine to cool fully before servicing the cooling system
- Remove the thermostat cover and inspect the thermostat cavity and sealing surfaces for corrosion or debris
- Install the replacement thermostat in the correct orientation per the Mercury service procedure
- Use a new gasket or seal on reassembly if specified
- Run the engine and verify normal operating temperature after installation
Technical FAQ
Why does the temperature rating matter?
Mercury uses different thermostat temperatures across its outboard lineup — 120°F, 125°F, 130°F, and 140°F ratings are all used on different engines. Installing the wrong temperature thermostat will cause the engine to run outside its designed operating range. Always match the temperature rating and part number to your specific engine.
What makes this a “self-cleaning” thermostat?
The self-cleaning design allows small particles and cooling-system debris to pass through the thermostat’s closing surfaces rather than build up on the seat. This reduces the risk of the thermostat sticking due to debris accumulation, which is particularly valuable in saltwater or silty operating environments.
Is this thermostat compatible with older 15/20 HP FourStroke engines?
No. 8M0139434 is specified for serial 0R833820 and above (2017 and newer). Earlier engines use a different thermostat configuration. Always confirm the serial number before ordering.
Parts Desk Insights
The most important fitment details for this thermostat are the 125°F rating and serial 0R833820 and above — these distinguish it from other Mercury outboard thermostats used on different engine families and configurations. A thermostat stuck open is one of the most common causes of an engine that never fully warms up or runs poorly at low speed, and it’s often overlooked because the engine doesn’t overheat. If a customer reports poor fuel economy or rough low-speed running on a 15/20 HP EFI, a stuck-open thermostat is worth checking before diving into fuel-system diagnosis. Browse the full Mercury Cooling System Parts collection for related thermostats, housings, and water pump components.
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 1-3 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
Returns & Satisfaction Guarantee
We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
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