Sierra 18-3406

Keep your Mercury Verado 4-stroke outboard running at peak performance with the Sierra 18-3406 thermostat assembly. Designed as a direct replacement for factory components, this thermostat regulates engine coolant temperature to prevent overheating and ensure smooth operation during long days on the water. Whether you're pushing through choppy waves or cruising calm bays, it delivers reliable temperature control that protects your investment.

Key Features That Set It Apart

• Precision Engineering: Manufactured to meet or exceed OEM specifications, ensuring a seamless fit and long-lasting durability in harsh marine environments.
• Optimal Temperature Regulation: Opens and closes at the exact points needed for your Verado model, maintaining ideal operating temps between 140-160°F to boost efficiency and reduce wear.
• Corrosion-Resistant Materials: Built with high-grade stainless steel and composites that stand up to saltwater exposure, freshwater corrosion, and constant vibration.
• Easy Gasket Integration: Includes all necessary seals for a leak-free install, minimizing downtime and frustration during maintenance.

Confirmed Compatibility for Verado Owners

This thermostat is tailored for select Mercury Verado 4-stroke engines, making it a go-to choice for boaters with these powerplants:

• 135/150/175 HP Verado (Serial Number 1B227000 and above)
• 200/225 HP Verado (Serial Number OT980000 and above)
• 250/275 HP Verado (Serial Number OT801000 and above)

Before installing, double-check your engine's serial number to confirm fitment—peace of mind starts with the right match. It's a smart upgrade for anyone noticing fluctuating temps, sluggish performance, or warning lights on the dash.

Signs Your Thermostat Needs Replacing

Spotting issues early can save your engine from serious damage. Watch for these common symptoms:

1. Engine running hotter or colder than usual, even after warmup.
2. Fluctuating temperature gauge readings during operation.
3. Reduced power output or hesitation under load.
4. Coolant leaks around the housing or hoses.
5. Overheating alarms triggered without obvious blockages.

Replacing proactively during routine service—like every 100-200 hours—keeps things reliable and avoids costly repairs down the line.

Pro Tips for Installation Success

Swapping out your thermostat is a straightforward DIY job that takes about 30-60 minutes with basic tools. Here's a quick rundown:

1. Drain the coolant from the closed system to avoid spills.
2. Remove the housing bolts and lift off the old unit—inspect for debris or old gasket residue.
3. Clean mating surfaces thoroughly for a perfect seal.
4. Install the new Sierra 18-3406 with fresh gaskets, torque to spec (usually 8-10 ft-lbs), and refill with Mercury-approved coolant mix.
5. Run the engine on muffs or in the water, checking for leaks and stable temps.

Pro tip: Always burp the system to remove air pockets, and monitor during the first few outings. If you're not comfortable wrenching, any certified marine tech can knock this out quickly.

Why Boaters Trust Sierra for Verado Maintenance

Sierra parts like the 18-3406 have earned a reputation among serious anglers, charter captains, and weekend warriors for their consistent quality and value. Unlike some generics that fail prematurely, this thermostat holds up season after season, helping you focus on the fish instead of fixes. Pair it with fresh coolant and a once-over of your water pump for comprehensive cooling system health.