MerCruiser Thermostat – 991551
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You have 30 days to request a return. All sale items are final sale.
The Mercury MerCruiser 991551 Thermostat is a genuine OEM 160°F (71°C) 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 160°F 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
- 160°F (71°C) 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: 160°F (71°C)
- Type: Engine thermostat
- Operation: Wax-element, spring-loaded valve
- Application: Closed cooling systems
What’s 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Alternate / Superseded Part Numbers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 MerCruisers—if your engine runs cold or throws fuel trim issues, this is usually the fix 👍
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 3–4 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
Returns & Satisfaction Guarantee
We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
We are serious about product integrity — and equally serious about taking care of our customers. If there is ever a product issue, contact us immediately so we can make it right.
For return authorization, please contact our support team before shipping anything back.