Mercury – Thermostat Assembly | Cooling System | 120°F Rated | 8M0090819

Overview

There’s a certain comfort that comes from an engine running at just the right temperature. You can hear it in the steady rhythm at idle and feel it when you push the throttle and the boat lifts effortlessly onto plane. That sense of control and predictability doesn’t happen by accident—it’s the result of dependable parts doing their job quietly beneath the cowling. The Mercury 8M0090819 Thermostat Assembly brings that peace of mind to every outing, keeping your 75–115 horsepower FourStroke running exactly the way it should.

Key Features

• Maintains a consistent 120-degree Fahrenheit operating temperature
• Designed for proper coolant flow in Mercury 2.1L EFI FourStroke outboards
• Durable, thermally precise construction for reliable long-term performance
• Direct-fit replacement engineered to Mercury Marine specifications
• Straightforward installation using common tools

Engine & Boat Compatibility

• Fits Mercury 75–115 horsepower 2.1L EFI FourStroke outboards
• Compatible with a wide range of hull styles, including pontoons, center consoles, aluminum fishing boats, and runabouts
• Ideal for freshwater and saltwater use

Performance & Practical Benefits

A thermostat may be a small piece of the cooling system, but it has a huge impact on how your engine behaves. With the 8M0090819 installed, your Mercury FourStroke warms quickly, holds a smooth idle, and avoids the sluggish feeling that comes from running too cool or too hot. The temperature regulation is steady, which helps improve combustion efficiency and protects internal components from unnecessary stress.

On the water, that translates into easy starts early in the morning, crisp throttle response when you’re navigating tight channels, and a dependable, steady feeling during long runs at mid-range speeds. The thermostat opens and closes with a tight, predictable motion, keeping the engine temperature dialed in no matter the load, the weather, or how long you’ve been running. And when you’re miles from the dock, that kind of reliability means everything.

Why Buy OEM

Cooling components must be precise—there’s simply no room for error. Mercury OEM thermostats are engineered to match the exact flow rate and temperature specifications your engine was designed for. Aftermarket versions often look similar, but the opening temperature can vary, which affects warm-up time, efficiency, and long-term reliability.

When you choose the genuine Mercury Marine part number 8M0090819, you’re choosing a thermostat that seats cleanly, fits without forcing, and behaves exactly the way the factory intended. It’s the smarter investment in the long run, protecting the heart of your engine and keeping your time on the water trouble-free.

FAQ

Q1: Which Mercury engines does this thermostat fit?

It’s designed specifically for Mercury 75–115 horsepower 2.1L EFI FourStroke outboards.

Q2: What makes this thermostat different from aftermarket options?

OEM thermostats open at the correct temperature and fit perfectly, ensuring proper coolant flow and consistent engine operation.

Q3: How do I confirm this is the correct part for my engine?

Match your serial number and model details with your owner’s manual to verify compatibility.

Q4: Can it be used in both saltwater and freshwater?

Yes—its materials and design tolerate both environments without issue.

Q5: Is installation straightforward?

Most boat owners can replace it themselves with basic tools, following the steps in the service manual.