The Mercury Mariner 130°F thermostat, Mercury OEM part number 8M0057307, is designed for many high-performance Mercury and Mariner V6 outboards using 3.0 Liter and 3.2 Liter cooling systems.
This thermostat is commonly used on Mercury EFI, DFI, XS, and Pro XS applications where proper engine temperature control is critical for idle quality, warm-up, fuel control, piston life, and overall cooling system performance.
A thermostat helps the engine reach and maintain the correct operating temperature by regulating water flow through the cylinder heads. When the engine is cold, the thermostat helps restrict coolant flow so the powerhead can warm up properly.
As temperature rises, the thermostat opens to allow additional cooling water through the system. On performance outboards such as Mercury 225, 250, 250XS, 300X, and 300XS models, correct thermostat operation helps stabilize temperature during idle, cruising, and hard running.
Why a 130°F Thermostat Matters
The 130° thermostat is commonly used in Mercury 3.0L and 3.2L V6 outboards because these engines need controlled warm-up and consistent cooling balance. A thermostat stuck open can make the engine run too cool, causing poor idle quality, longer warm-up time, rich running, carbon buildup, and poor fuel efficiency.
A thermostat stuck closed or restricted by debris can cause overheating, warning alarms, power reduction, piston damage, or severe powerhead failure.
For Mercury EFI, DFI, Optimax, XS, and Pro XS outboards, stable temperature is especially important because the fuel and ignition systems rely on accurate engine operating conditions. Keeping the correct Mercury 8M0057307 thermostat in place helps the engine run as designed.
Common Fitment
This Mercury 130°F thermostat fits many Mercury and Mariner V6 outboards from 200 HP through 300 HP, depending on model and serial number. Common applications include Mercury 225 HP, Mercury 250 HP, Mercury 250XS, Mercury 300X, Mercury 300XS, 3.0 Liter V6, 3.2 Liter V6, EFI, DFI, XS, and Pro XS models.
Fitment should always be confirmed by engine serial number before ordering, because Mercury used different cooling components across model years, horsepower ratings, and engine families.
Cross Reference Part Numbers
This thermostat is commonly searched under Mercury OEM part number 8M0057307 and cross-reference numbers 13610, 885599, 885599002, 885599003, 885599004, 885599005, and 8855991.
Customers may also search for this part as a Mercury 130 degree thermostat, Mercury 3.0L thermostat, Mercury 3.2L thermostat, Mercury Pro XS thermostat, Mercury 300X thermostat, Mercury 300XS thermostat, or Mercury Mariner V6 thermostat.
When to Replace Your Thermostat
Replace the thermostat during cooling system service, after overheating, when debris is found in the cooling passages, when the engine runs too cold, when the engine overheats at idle, or when temperature readings become inconsistent.
If one thermostat is contaminated, corroded, or sticking, inspect the rest of the cooling system, including poppet valve components, water pump, hoses, adapters, cylinder head covers, and thermostat housings.
Many Mercury V6 engines use one thermostat per cylinder head, so a complete service may require two thermostats. Always verify the quantity required for your exact engine before ordering.
Installation Notes
Install the Mercury 8M0057307 thermostat only on a cool engine. Remove the thermostat cover carefully, clean the housing surface, inspect for corrosion or debris, and make sure the thermostat seats correctly in the housing.
Replace any required seals, gaskets, or O-rings during service. Do not use excessive sealant, and always follow the Mercury service manual torque specifications for your exact engine.
After installation, run the engine on a proper water supply and verify normal water flow, stable temperature, and no leaks. On high-performance Mercury outboards, monitor both water pressure and temperature because one does not always confirm the other.
Good water pressure with a stuck thermostat can still create cooling problems, and a good thermostat cannot compensate for a weak water pump or restricted cooling passage.
Buckshot Racing #77 Tech Note
Do not remove thermostats as a shortcut to cooling problems. Mercury 3.0L and 3.2L V6 outboards are designed to operate with controlled cooling flow. Running without the correct thermostat can cause poor warm-up, unstable temperature, uneven cylinder cooling, and tuning issues.
For reliable performance, use the correct 130°F thermostat, keep the cooling system clean, and service the water pump and poppet system as part of regular maintenance.
Whether you are servicing a Mercury 225, freshening a 250XS, maintaining a 300X, or rebuilding a 300XS Pro XS-style cooling system, the Mercury OEM 8M0057307 130° thermostat is an important part of keeping your V6 outboard at the correct operating temperature.
Built for Mercury and Mariner 3.0 Liter and 3.2 Liter applications, this thermostat helps protect your powerhead, improve warm-up, and support dependable performance on the water.