Description

The Mercury / Quicksilver 8M0090819 Thermostat Assembly is a genuine OEM 120°F thermostat with integrated housing, designed for Mercury EFI 4-stroke outboards.

Unlike a standard thermostat, this is a complete assembly that includes both the thermostat and its housing, ensuring proper sealing, alignment, and coolant flow. It regulates engine temperature by controlling water flow through the powerhead, allowing the engine to warm up quickly and maintain stable operating temperature under load.

The 120°F calibration is critical for these engines, supporting proper fuel mapping, combustion efficiency, and corrosion control in raw water cooling systems. Built to Mercury Marine specifications, it provides consistent thermal response, long service life, and exact fitment.

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Key Features

• Genuine Mercury / Quicksilver OEM assembly
• 120°F (49°C) temperature rating
• Complete unit: thermostat + housing
• Designed for raw water cooled outboards
• Maintains proper engine operating temperature
• Ensures correct coolant flow and sealing
• Direct OEM replacement (no modification required)

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Engine Compatibility

Mercury 4-Stroke EFI Outboards (2.1L Platform)

Fits:

• 75 HP
• 80 HP
• 90 HP
• 100 HP
• 115 HP

Also used on:

• EFI SeaPro models
• EFI Jet drive variants

Designed for:

• 2.1L 4-cylinder EFI FourStroke engines

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Important Fitment Notes

• Raw water cooled systems only
• Must match 120°F spec for proper operation
• Not compatible with Verado engines
• Includes housing – not interchangeable with thermostat-only designs

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Specifications

• Part Number: 8M0090819
• Temperature Rating: 120°F (49°C)
• Type: Thermostat assembly (with housing)
• Cooling System: Raw water cooled
• Engine Platform: Mercury 2.1L EFI 4-stroke
• Construction: Marine-grade composite/metal housing

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What’s Included

• Thermostat
• Thermostat housing assembly

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When This Part Needs Replacement

• Engine overheating or running hot
• Engine running too cold (stuck open)
• Coolant leaks at thermostat housing
• Corrosion or damage to housing
• Routine maintenance on high-hour engines

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Installation Notes

• Replace as a complete assembly (do not reuse old housing)
• Inspect sealing surfaces and hoses
• Torque evenly to avoid housing distortion
• Verify proper operating temperature after install

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Alternate / Superseded Part Numbers

OEM Cross References

• Mercury / Quicksilver: 8M0090819
• Superseded by: 8M0204728

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Shipping

Shipping: Ships same or next day for fast Canada wide shipping.

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Parts Desk Insight

If you’re seeing leaks around the thermostat area, replace the whole assembly—not just the thermostat. These housings warp and corrode over time