MerCruiser Thermostat 991551

The Mercury MerCruiser 991551 Thermostat is a genuine OEM 160F (71C) thermostat designed for GM-based MerCruiser marine engines equipped with closed cooling (freshwater cooling) systems.

This thermostat regulates coolant flow between the engine and heat exchanger, allowing the engine to reach and maintain optimal operating temperature. The 160F calibration is critical for proper combustion efficiency, fuel management, and engine longevity on MPI and EFI MerCruiser engines.

It uses a wax-element expansion valve, which opens progressively as temperature rises, ensuring stable coolant flow under varying engine loads. Built to Mercury Marine specifications, it delivers consistent opening characteristics, durability, and exact fitment.

Key Features

• Genuine Mercury / Quicksilver OEM part
• 160F (71C) temperature rating
• Designed for closed cooling systems only
• Maintains proper engine operating temperature
• Improves fuel efficiency and combustion stability
• Prevents overheating and cold-running conditions
• Direct OEM replacement

Engine Compatibility

MerCruiser GM-Based Engines (1998+)

Commonly used on:

• 4.3L V6 (262 CID)
• 5.0L V8 (305 CID)
• 5.7L V8 (350 CID)
• 6.2L V8 (MPI / Horizon)
• 7.4L / 8.2L (454 / 502 MAG MPI)

Fits engines with:

• MPI / EFI fuel systems
• Alpha & Bravo sterndrive configurations
• Closed cooling (freshwater cooling) systems

Important Fitment Notes

• Designed for closed cooling systems (FWC)
• Not for raw water cooled engines requiring lower temp thermostats
• Common on 1998+ MerCruiser engines

Specifications

• Part Number: 991551
• Temperature Rating: 160F (71C)
• Type: Engine thermostat
• Operation: Wax-element, spring-loaded valve
• Application: Closed cooling systems

Whats Included

• Thermostat (gasket sold separately unless specified)

When This Part Needs Replacement

• Engine not reaching proper temperature
• Engine overheating under load
• Temperature fluctuations
• Poor fuel efficiency or rough running

Installation Notes

• Install with correct orientation (spring toward engine)
• Replace gasket/seal during installation
• Clean housing and sealing surfaces
• Verify operating temperature after install

Alternate / Superseded Part Numbers

OEM Cross References

• Mercury / MerCruiser: 991551
• Quicksilver equivalent: 991551

Shipping

Shipping: Ships same or next day for fast Canada wide shipping.

Parts Desk Insight

This is the standard 160 thermostat for MPI MerCruisersif your engine runs cold or throws fuel trim issues, this is usually the fix