Sierra 18-3549 Thermostat 120°F – Johnson Evinrude OMC Replacement
Free returns on all eligible orders
You have 30 days to request a return. All sale items are final sale.
The Sierra 18-3549 Thermostat (120°F) is a premium aftermarket replacement designed for Johnson, Evinrude, and OMC outboard engines. It regulates engine temperature to ensure proper warm-up and consistent cooling system performance in marine environments.
Calibrated to 120°F (49°C), this thermostat is ideal for 2-stroke outboard applications, helping maintain optimal operating temperature while preventing overheating and improving engine efficiency. Built to meet or exceed OEM specifications, it delivers dependable performance in both freshwater and saltwater use.
Key Features
- Sierra premium aftermarket replacement
- 120°F (49°C) temperature rating
- Maintains proper engine operating temperature
- Improves cooling system efficiency
- Corrosion-resistant marine construction
Engine Compatibility
Johnson / Evinrude / OMC Outboards
Commonly used on:
- 2HP – 40HP outboards
- Many 2-stroke Johnson & Evinrude models
- OMC small to mid-range engines
➡️ Widely used across older carbureted outboard platforms
Serial Number Fitment Notice
Fitment depends on:
- Engine model and year
- Cooling system design
⚠️ Always confirm compatibility using your engine model and serial number before ordering.
Specifications
- Part Number: 18-3549
- Temperature Rating: 120°F (49°C)
- Type: Engine thermostat
- Function: Regulates coolant temperature
- Construction: Marine-grade metal housing
What’s Included
- Thermostat (gasket/seal sold separately unless specified)
When This Part Needs Replacement
Replace the thermostat if you notice:
- Engine overheating
- Engine running too cold
- Poor idle performance
- Inconsistent temperature readings
Installation Notes
- Replace gasket or seal during installation
- Ensure proper orientation
- Inspect housing for corrosion or debris
- Test cooling system after install
Alternate / Superseded Part Numbers
OEM Cross References
- OMC / Johnson / Evinrude: 384891 / 5005440
Aftermarket Cross References
- Sierra: 18-3549
Shipping
Shipping: Ships same or next day for fast Canada wide shipping. ⚓
💡 Parts Desk Insight
On older Johnson/Evinrude 2-strokes, a stuck-open thermostat will make the engine run cold and rough—this is one of the easiest fixes 👍
.
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.