Great Lakes Memories Thermostat Kit for Mercury Mercruiser 2.5L/3.0L — 140°F Replacement with Gasket

Great Lakes Memories thermostat kit is a complete replacement thermostat and gasket set engineered to restore correct operating temperature on Mercury Mercruiser 2.5L and 3.0L inline engines.

This replacement kit is designed for small Mercruiser four-cylinder marine engines and fits many inline-six applications as well. It includes a 140°F thermostat and a sealing gasket manufactured to meet or exceed original equipment specifications, making it a direct swap for part numbers 59078, 59078Q3 and 18-3650. The kit is intended to help the engine warm up predictably and maintain stable cooling system performance when installed according to manufacturer service guidance.

Will this thermostat kit fit my Mercruiser 2.5 or 3.0 engine?

Yes — the kit was developed as a direct replacement for common Mercury Mercruiser inline 2.5L and 3.0L four-cylinder units and is compatible with many inline-six applications that use the same thermostat specification. Verify fitment by comparing part numbers and the thermostat housing design before ordering.

• Replaces OEM part numbers 59078, 59078Q3, and 18-3650
• Intended for Mercruiser 2.5L and 3.0L inline engines
• Also compatible with several inline-six engine models using the same thermostat size
• Confirm housing and bolt pattern to ensure proper fit

What temperature does the thermostat open, and why does that matter?

The thermostat in this kit is a 140°F (approximately 60°C) unit, chosen to match factory operating ranges for the specified engines. A thermostat opening at the correct temperature helps the engine reach operating range quickly and keeps coolant flow regulated once warmed, supporting consistent engine performance and emissions behavior.

• 140°F rated opening temperature for predictable warm-up
• Maintains stable coolant circulation after warm-up
• Helps protect against overcooling or premature warm-up issues
• Meets or exceeds OEM thermostatic performance specifications

What comes in the kit and how does it simplify installation?

The package contains a 140°F thermostat and a matching gasket, providing the essential components needed for a typical thermostat replacement service. Including the correct gasket reduces the chance of leaks and saves you from sourcing separate sealing parts, streamlining the maintenance task.

• One 140°F thermostat sized for Mercruiser inline engines
• One matching gasket to ensure a proper seal
• Sold as a single kit for immediate replacement use
• Designed to meet OEM fit and function for straightforward swap-in

Who is this for? This thermostat kit is built for marine owners, DIY mechanics, and professional technicians working on Mercury Mercruiser 2.5L or 3.0L inline engines who need a reliable OEM-replacement thermostat and gasket. It’s aimed at anyone replacing a failed thermostat, correcting irregular engine temperatures, or performing scheduled cooling-system maintenance who requires parts that match original specifications and simplify installation.