Sierra 18-3667
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Within 30 days
Sierra 18-3667 Product Description
Keep your Mercury outboard running cool and strong with the Sierra 18-3667 thermostat kit, a reliable aftermarket match for OEM part 99155T-2. Designed specifically for demanding marine environments, this kit helps maintain optimal engine temperature, preventing overheating during long days on the water. Boaters trust Sierra for parts that deliver OEM-level performance without the premium price tag, making it a smart choice for routine maintenance or unexpected repairs.
Engineered for Peak Performance
Marine engines face constant exposure to saltwater, vibration, and heat cycles, which can wear out thermostats quickly. The Sierra 18-3667 steps in with a precision-calibrated thermostat that opens and closes at the right moments—typically around 140°F for balanced cooling. Paired with high-quality gaskets and seals, it ensures a leak-free seal, promoting efficient coolant flow and extending engine life. Forget about erratic idling or steam rising from your motor; this kit restores smooth operation and peace of mind.
Standout Features Boaters Love
- Durable Construction: Brass thermostat body resists corrosion, outlasting cheaper alternatives in harsh saltwater conditions.
- Complete Kit: Includes thermostat, O-rings, and gaskets for a hassle-free replacement—no hunting for extras.
- Exact OEM Specs: Matches Mercury 99155T-2 dimensions and function for plug-and-play reliability on compatible 4-cylinder models.
- Temperature Control: Fine-tuned for 55-40 applications, keeping your engine in the sweet spot whether trolling or wide-open throttle.
- Easy Monitoring: Supports standard temp gauges, so you always know your engine's status.
Seamless Compatibility
This thermostat kit fits a range of Mercury outboards, particularly inline 4-cylinder engines from the mid-80s through the 2000s. It's ideal for models requiring the 99155T-2 spec, like certain 90-125 HP variants. Always cross-check your engine's serial number to confirm, but Sierra's engineering ensures it bolts right in without modifications. Pro tip: Pair it with fresh coolant and a flush for maximum longevity.
Pro Installation and Maintenance Tips
Swapping in the Sierra 18-3667 takes about 30-45 minutes with basic tools. Drain the coolant, remove the housing bolts, pop out the old unit, clean the mating surfaces thoroughly, and install the new kit with a light coat of marine sealant on the gasket. Torque to spec (usually 8-10 ft-lbs), refill, and test-run at idle. Common pitfalls? Skipping the cleaning step leads to leaks—always flush debris. For upkeep, inspect annually or every 100 hours; early replacement avoids costly head gasket failures.
Bonus Maintenance Advice
- Flush the cooling system yearly to remove salt buildup.
- Monitor for sticky thermostats by feeling hoses—hot on top, cool on bottom means it's working.
- Store spares onboard for trailering peace of mind.
Why Sierra Beats the Rest
Sierra has built a reputation among mechanics and weekend warriors for marine parts that punch above their weight. Unlike generic imports that fail prematurely, Sierra uses rigorous testing to mimic Mercury's quality. Real-world feedback highlights fewer comebacks and consistent cooling, even in tournament fishing or coastal cruising. Invest in this kit, and you're backing your engine with parts proven on thousands of boats.
Frequently Asked Questions
What engines is the Sierra 18-3667 thermostat kit compatible with?
It replaces Mercury 99155T-2 and fits many inline 4-cylinder outboards, especially 90-125 HP models from the 1980s to 2000s. Verify by your engine's serial number for the perfect match.
Does this kit include everything needed for installation?
Yes, it comes complete with the thermostat, gaskets, and O-rings—ready for a straightforward swap without additional parts.
How do I know if my thermostat is failing?
Signs include overheating at idle, fluctuating temps, or one cooling hose much hotter than the other. Test by removing and submerging in hot water to check opening.
What's the temperature rating for this Sierra thermostat?
Calibrated for 55-40 applications, it typically opens around 140°F to provide reliable cooling across operating ranges.
Can I install this myself, or do I need a mechanic?
Most DIYers handle it easily with wrenches and a drain pan. Just clean surfaces well and follow torque specs to avoid issues.
How often should I replace the thermostat?
Every 2-3 years or 100-200 hours of use, especially in saltwater, to prevent unexpected breakdowns.
Is Sierra as good as genuine Mercury parts?
Absolutely—Sierra matches OEM specs with proven durability, trusted by pros for cost savings without sacrificing quality.
Save $5 when you buy the Sierra 18-3667 for only $23 from NVN Marine! Same day shipping before 3pm.
Sierra 18-3667 Technical Specifications
| Type: | |
|---|---|
| Vendor: |
Sierra Engine Parts |
| Weight: |
0.16 lb |
Sierra 18-3667 User Reviews
Sierra 18-3667 Shipping & Return
| Shipping Weight: |
0.16 lb |
|---|---|
| Box Sizes: |
1.00 (h) x 6.00 (w) x 7.00 (l) |
| Cubic Feet: |
42.0 |
| Returnable: |
Yes |
For detailed shipping policy please check our Shipping Policy and you can find our Return and Refund Policy here.