MerCruiser Closed Cooling Thermostat Kit 807252T4

Closed Cooling Thermostat Kit Part 807252T4

This is the genuine OEM thermostat kit for MerCruiser GM-based engines equipped with closed cooling (freshwater cooling) systems. The kit is rated at the higher 170F180F range required for closed cooling operation not the 143F or 160F thermostats used in raw water cooled engines. Running the wrong temperature thermostat in a closed cooling engine causes improper warm-up, reduced efficiency, and can shorten engine life.

The kit includes everything needed for a complete thermostat service: the thermostat, sealing gaskets, and installation hardware, so no separate gasket sourcing is required.

Supersedes / Part Number Reference

• Current OEM number: 807252T4
• Supersedes: 807252T3, 807252Q4
• Aftermarket cross-reference: Sierra 18-3648

Compatible Applications

For closed cooling (freshwater / heat exchanger) equipped engines only. Not for standard raw water cooled engines. Confirm by engine serial number and cooling system configuration.

• 4.3L / 262 CID V6 Closed cooling models with heat exchanger
• 5.0L / 305 CID V8 Closed cooling models with heat exchanger
• 5.7L / 350 CID V8 Closed cooling models, including Bravo sterndrive and MIE inboard configurations
• 7.4L / 454 CID V8 Closed cooling models with heat exchanger
• 8.2L / 502 CID V8 Closed cooling models with heat exchanger

Serial Number Notice

Fitment varies by engine serial number and cooling system configuration. Engines converted from raw water to closed cooling may require a different thermostat kit. Confirm your cooling system type and engine serial number before ordering.

When This Part Needs Replacement

• Engine is running too hot or overheating
• Engine is not reaching or maintaining consistent operating temperature
• Thermostat is sticking open or closed
• Corrosion is present on the thermostat or housing
• Cooling system service or rebuild is underway
• Scheduled preventative maintenance on a closed cooling system

Installation Notes

• Use all included gaskets and seals do not reuse old ones
• Install the thermostat with the spring side facing toward the engine (inward)
• Clean the thermostat housing and all sealing surfaces before installation
• Inspect the heat exchanger and coolant passages for blockage or scale buildup
• Check coolant condition and replace if contaminated or overdue
• Follow the applicable MerCruiser service procedure for torque specifications

Parts Desk Insights

Most common customer issue: Installing a raw water thermostat (typically 143F or 160F) into a closed cooling engine. Closed cooling systems require the higher 170F180F thermostat using the wrong one causes the engine to run too cool, which increases fuel consumption, accelerates wear, and can prevent the engine from reaching proper operating efficiency. If youre unsure which cooling system your engine has, check for a heat exchanger mounted to the engine if it has one, you need this kit.

Best use case: Ideal for scheduled thermostat replacement on closed cooling MerCruiser V6 and V8 engines, cooling system overhauls, or any time the thermostat housing is opened for service. The kit format means no separate gasket hunting everything for a complete installation is included.