GHmarine Thermostat Kit for MerCruiser 2.5L & 3.0L — 140°F Cartridge with Gasket (59078Q3)

If your MerCruiser sterndrive is running hotter or you need a factory-style replacement, this GHmarine kit includes the thermostatic element and the matching cover gasket to restore proper coolant flow and engine temperature regulation. Designed to fit a wide range of MerCruiser MCM GM 4/6 cylinder and 120/140/165 drive models, it replaces multiple OEM and aftermarket part numbers so you can install it with confidence.

Which MerCruiser models does this thermostat kit fit?

This thermostat kit is tailored for MerCruiser sterndrive engines in 2.5L and 3.0L displacements and covers several drive variants. It fits MCM GM 4- and 6-cylinder configurations as well as 120 GM, 140 GM, and 165 GM drive families. Use the cross-reference list to confirm compatibility before purchase.

• Compatible with MerCruiser 2.5L and 3.0L sterndrive engines
• Fits MCM GM 4- and 6-cylinder, 120/140/165 drive models
• Cross-replaces MerCruiser 59078Q3 and related OEM numbers
• Works with common aftermarket equivalents (GLM, Mallory, Sierra)

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

The thermostat opens at 140° F, which is a standard operating threshold for many MerCruiser sterndrives. Maintaining that opening point helps the engine reach and hold its intended operating temperature for efficient combustion and corrosion control without overcooling. Selecting the correct temperature rating prevents poor warm-up behavior or overheating risks associated with wrong-spec parts.

• Temperature rating: 140° F (opens at design point)
• Ensures consistent coolant flow when engine reaches operating heat
• Helps avoid extended warm-up times or premature thermostat opening
• Matches common factory specifications for the listed models

What comes in the GHmarine thermostat kit and are installation parts included?

The package contains the thermostatic cartridge assembly (59078) and a replacement thermostat cover gasket (47590). That means you have the two primary items needed for a simple gasket-and-thermostat swap—no extra seals or housings are required in most installs. Always inspect the cover, housing, and cooling passages for wear or corrosion during replacement.

• Included: thermostat element part 59078 and cover gasket 47590
• Direct-fit replacement for listed MerCruiser and aftermarket numbers
• Install typically requires basic hand tools and cooling system drain
• Check mating surfaces and replace corroded hardware as needed

Who is this for: The GHmarine 59078Q3 Thermostat Kit is aimed at DIY boat owners, marine mechanics, and service shops who maintain MerCruiser 2.5L and 3.0L sterndrive engines and need a reliable, OEM-equivalent thermostat and gasket. It’s suitable for routine cooling system service, end-of-season rebuilds, or when troubleshooting temperature regulation. If you’re matching cross-reference numbers or restoring factory-like performance in compatible MerCruiser models, this kit provides the core replacement parts to get the cooling system back to spec.