Mercury – Thermostat | Cooling System | Factory Marine Finish | 991551

Overview

There’s a certain comfort in watching your engine settle into that perfect operating temperature—steady, calm, and exactly where it should be. It’s the feeling you get when you’ve been on the water long enough to know when something is right, and when something isn’t. A good thermostat doesn’t shout for attention; it simply keeps the heart of your boat running smooth and predictable. That quiet reliability is why so many boaters trust Mercury Marine parts like the 991551 thermostat.

Key Features

• Maintains precise 160-degree Fahrenheit engine operating temperature
• Designed for MerCruiser V6 and V8 General Motors–based marine engines
• Built with durable marine-grade materials for long-term temperature stability
• Factory-engineered to promote efficient coolant flow and consistent heat regulation
• Direct replacement designed for easy installation using common tools
• Supports closed-cooling systems requiring a 160-degree thermostat

Engine & Boat Compatibility

• Fits 1998 and newer MerCruiser marinized V6 and V8 engines
• Compatible with closed-cooling systems built on General Motors engine platforms
• Ideal for cruisers, sport boats, runabouts, bowriders, and family recreation boats
• Suitable for engines that depend on steady operating temperatures for fuel efficiency and long-term durability

Performance & Practical Benefits

A thermostat might be small, but you’ll feel its absence immediately when it’s not doing its job. With the Mercury Marine part 991551 installed, your engine reaches its ideal temperature quickly and stays there. That means smoother idling, cleaner combustion, and more predictable power when you lean into the throttle. Everything feels a bit steadier—no wandering temperatures, no unnecessary strain, no guessing whether your cooling system is keeping up.

Because this thermostat is built specifically for marine use, it handles the constant changes in workload and water temperature that boats experience. The fit is tight and secure, sitting exactly where it should without fighting alignment or installation. Once it’s in place, the engine responds with a more confident rhythm. Your fuel system, ignition timing, and cooling passages all work together more efficiently when the temperature remains stable, and this thermostat makes that happen every time you head out.

For boaters who spend long days cruising or towing, that stability translates to better performance and longer engine life. And when you’re miles from the ramp, that peace of mind is worth more than any spec sheet can say.

Why Buy OEM

There’s a big difference between a thermostat that simply fits and one that’s engineered for your exact engine. With an OEM Mercury Marine part, you’re getting the proper temperature rating, the correct response time, and the durability expected from factory-grade components. Aftermarket parts often fall short in consistency, which can lead to uneven heating, poor performance, or premature failure.

The Mercury Marine part number 991551 is designed to match your engine’s cooling system perfectly. It’s reliable, cost-effective over the long haul, and delivers the kind of performance you only get from original equipment. When you want your engine to run right the first time—and every time—OEM is the way to go.

FAQ

Q1: Which engines does the 991551 thermostat fit?

It fits 1998 and newer MerCruiser marinized V6 and V8 engines with closed-cooling systems requiring a 160-degree thermostat.

Q2: How is this different from aftermarket versions?

This thermostat is engineered to Mercury’s strict specifications, offering precise temperature control and long-term reliability.

Q3: How do I confirm it’s the correct replacement?

Check your engine’s parts listing or owner’s manual for the exact part number required by your cooling system.

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

Yes, it’s built for both and designed to withstand harsh marine conditions.

Q5: Is installation straightforward?

Most owners can replace it using basic tools by following the procedure outlined in their service manual.