Sierra 18-3555
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Sierra 18-3555 Product Description
Keep your outboard engine running at peak efficiency with the Sierra 18-3555 thermostat, a reliable aftermarket match for the OEM Mercury 99155A1. Designed for demanding marine conditions, this thermostat ensures optimal coolant flow to prevent overheating during long days on the water. Boat owners trust it for its precise temperature regulation and durable build that stands up to corrosion and vibration.
Key Features and Performance Benefits
- Exact OEM Specifications: Matches the Mercury 99155A1 in fit, form, and function, opening at the precise temperature to maintain ideal engine operating conditions.
- Corrosion-Resistant Materials: Built with high-quality brass and stainless steel components to resist saltwater exposure and extend service life.
- Seamless Integration: Direct replacement that installs without modifications, getting you back on the water faster.
- Enhanced Engine Protection: Prevents thermal shock by regulating coolant flow, reducing wear on cylinder heads and gaskets.
- Complete Kit Ready: Includes necessary seals and hardware for a hassle-free swap, minimizing downtime.
Whether you're prepping for fishing trips or weekend cruises, this thermostat delivers consistent performance that supports fuel efficiency and smoother operation.
Engine Compatibility Guide
The Sierra 18-3555 fits a range of Mercury and Mariner 4-cylinder outboards, commonly found in 90-125 HP models from the late 1980s through early 2000s. Always cross-reference your engine's serial number to confirm fitment—it's a popular choice for popular carbureted and EFI setups. Pro tip: Check your service manual for the exact housing location, typically inline with the cooling system manifold.
| Brand | Model Range | HP |
|---|---|---|
| Mercury/Mariner | Inline 4-cylinder | 90-125 HP |
| Mercury/Mariner | Black Max/Sport Jet | Similar series |
Easy Installation Steps and Pro Tips
- Prep Work: Shut off fuel, disconnect battery, and drain coolant to avoid spills.
- Access the Housing: Remove the thermostat cover bolts—usually 4-6 on the cylinder head or manifold.
- Swap and Seal: Extract the old unit, clean mating surfaces, install the new Sierra thermostat with included gasket, and torque to spec (around 10-15 ft-lbs).
- Refill and Test: Top off with fresh coolant mix, bleed air from the system, and run the engine to verify operation.
- Bonus Tip: While in there, inspect hoses and impellers for wear to prevent future issues.
Most DIYers complete this in under an hour with basic tools. If you're new to outboard maintenance, watch for the telltale signs of a failing thermostat like erratic temps or steam from the exhaust.
Long-Term Maintenance for Reliability
Thermostats don't last forever—flush your cooling system annually and inspect during tune-ups. Common culprits for failure include debris buildup or old coolant degrading seals. Pair this replacement with a quality flush kit for maximum longevity. Users report this Sierra part outperforming generics in high-heat scenarios, keeping temps steady even under full throttle.
Frequently Asked Questions
What engines is the Sierra 18-3555 thermostat compatible with?
It fits Mercury and Mariner 4-cylinder outboards in the 90-125 HP range, matching OEM 99155A1 specs. Verify with your engine serial number.
Does the Sierra 18-3555 include installation hardware?
Yes, it comes as a complete kit with gaskets and O-rings for straightforward replacement.
What temperature does this thermostat open at?
It opens precisely at the OEM-specified temperature, typically around 140-150°F, for optimal engine cooling.
How do I know if my thermostat needs replacing?
Look for overheating, fluctuating gauges, or poor idle performance. Test by removing and placing in hot water to check opening.
Is the Sierra 18-3555 suitable for saltwater use?
Absolutely—its corrosion-resistant materials make it ideal for brackish and saltwater environments.
Can I install this thermostat myself?
Yes, with basic tools and mechanical know-how. Follow the steps in your service manual for best results.
What's the benefit of choosing Sierra over other aftermarket brands?
Sierra offers OEM-level quality at a better value, with proven reliability in marine applications.
Save $3 when you buy the Sierra 18-3555 for only $20 from NVN Marine! Same day shipping before 3pm.
Sierra 18-3555 Technical Specifications
| Type: | |
|---|---|
| Vendor: |
Sierra Engine Parts |
| Weight: |
0.07 lb |
Sierra 18-3555 User Reviews
Sierra 18-3555 Shipping & Return
| Shipping Weight: |
0.07 lb |
|---|---|
| Box Sizes: |
2.00 (h) x 6.00 (w) x 7.00 (l) |
| Cubic Feet: |
84.0 |
| Returnable: |
Yes |
For detailed shipping policy please check our Shipping Policy and you can find our Return and Refund Policy here.