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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.
Whether you're prepping for fishing trips or weekend cruises, this thermostat delivers consistent performance that supports fuel efficiency and smoother operation.
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 |
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.
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.
It fits Mercury and Mariner 4-cylinder outboards in the 90-125 HP range, matching OEM 99155A1 specs. Verify with your engine serial number.
Yes, it comes as a complete kit with gaskets and O-rings for straightforward replacement.
It opens precisely at the OEM-specified temperature, typically around 140-150°F, for optimal engine cooling.
Look for overheating, fluctuating gauges, or poor idle performance. Test by removing and placing in hot water to check opening.
Absolutely—its corrosion-resistant materials make it ideal for brackish and saltwater environments.
Yes, with basic tools and mechanical know-how. Follow the steps in your service manual for best results.
Sierra offers OEM-level quality at a better value, with proven reliability in marine applications.
Ships within 48 hours · Estimated delivery Sep 19 - Sep 24
US$40
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