Mercury – Thermostat Kit | Cooling System Part | Inline 4 & 6 GM Engines | 59078T3

Overview

Every engine runs better when it’s running at the right temperature. The Mercury Marine 59078T3 Thermostat Kit helps older GM-built MerCruiser inline 4- and 6-cylinder engines warm up quickly and stay at a steady 140°F — the sweet spot for smooth performance and long life. This kit includes both the thermostat and cork gasket, so you can replace everything at once and get back on the water fast. Built tough from corrosion-resistant metal, it’s designed to handle years of heat, vibration, and marine exposure without sticking or leaking.

Key Features

• Purpose: Keeps your MerCruiser running at a steady temperature for top performance.
• Contents: Comes with a 140°F thermostat and a cork gasket for a complete replacement.
• Material: Made from marine-grade metals that resist corrosion and pitting.
• Performance: Helps the engine warm up fast, hold temperature under load, and avoid overheating.
• Durability: Handles tough marine conditions without warping or binding.
• Ease of Use: Installs easily with hand tools — simple, fast, and reliable.

Engine & Boat Compatibility

• Older GM-built MerCruiser inline 4-cylinder engines
• Older GM-built MerCruiser inline 6-cylinder engines

Performance & Real-World Benefits

A thermostat may not be the most exciting part on your engine, but it’s one of the most important. The 59078T3 kit helps your cooling system stay balanced — not running too cold and not too hot. The 140°F thermostat makes sure your engine reaches the right temperature quickly, improving efficiency and protecting against internal wear.

The included cork gasket seals tight, preventing leaks and pressure loss that can lead to overheating. Swapping your thermostat regularly keeps your cooling system in top shape, giving you peace of mind every time you start the engine.

Why Choose Mercury OEM

The Mercury 59078T3 is the same quality part Mercury installs at the factory — built, tested, and proven for marine use. Aftermarket thermostats might open too early, run at the wrong temperature, or corrode before the season’s over.

Mercury’s OEM design ensures accurate temperature control, a perfect fit, and long-lasting reliability. It’s a small, affordable part that helps your older MerCruiser run smoother and last longer.

FAQ

Q1: Which engines does this thermostat kit fit?
It fits older GM-built MerCruiser inline 4- and 6-cylinder engines.

Q2: What’s included?
The kit includes a 140°F thermostat and cork gasket for a leak-free install.

Q3: What are the signs it needs replacing?
If your engine overheats, runs cold, or fluctuates in temperature, it’s time to change it.

Q4: Can I replace it myself?
Yes — it’s an easy job with basic tools and your service manual.

Q5: Why OEM instead of aftermarket?
OEM thermostats fit right, last longer, and maintain the temperature range Mercury designed for your engine.