Sierra 18-3378 391635 Johnson/Evinrude Water Pump

Keep your Johnson or Evinrude outboard running cool under pressure with the Sierra 18-3378 water pump kit, a reliable replacement for OEM part 391635. Designed for marine enthusiasts who demand durability on the water, this pump ensures optimal coolant flow to prevent overheating during long runs or high-RPM operation. Whether you're tackling rough waves or leisurely cruises, it delivers consistent performance without the hassle of frequent replacements.

Key Features That Stand Out

• High-Quality Impeller: Nitrile rubber construction resists heat, oil, and chemicals for extended service life in harsh saltwater environments.
• Complete Kit: Includes impeller, housing, lower unit wear plate, O-rings, gaskets, and cup washer—everything needed for a straightforward rebuild.
• Precision Engineering: Matches OEM specifications for seamless fit and flow rates, restoring factory-level cooling efficiency.
• Corrosion-Resistant Materials: Stainless steel and bronze components battle galvanic corrosion common in marine settings.

Perfect Fit for Your Engine

This Sierra pump fits a range of Johnson and Evinrude models from 65HP to 125HP, typically 1988-2001 V4 and inline engines. Always verify your lower unit setup against service manuals, as variations exist across gearcases. It's engineered for both freshwater and saltwater use, making it versatile for diverse boating conditions.

Step-by-Step Installation Insights

Replacing your outboard's water pump is a weekend warrior project that boosts reliability. Here's a pro tip-filled guide:

1. Prep Work: Disconnect the battery, tilt the engine up, and drain the gear lube to access the lower unit.
2. Disassemble: Remove the gearcase bolts, slide off the lower unit, and inspect the old impeller for glazing or chunks—common failure signs.
3. Swap Components: Lubricate the new impeller with marine grease, twist it into the housing (avoid petroleum-based lubes), and align the wear plate precisely.
4. Reassemble and Test: Torque bolts to spec, refill lube, and run the engine on muffs. Check for strong pee stream from the tell-tale—aim for steady flow at idle and WOT.

Pro Tip: Time your replacement every 2-3 years or 300 hours, especially if you spot rubber bits in the pee stream or erratic temps.

Why This Pump Excels in Real-World Use

Overheating isn't just inconvenient—it's a engine killer. A worn pump restricts flow, spiking temps and risking piston seizure. Sierra's 18-3378 restores full circulation, dropping operating temps by up to 20°F in tests. Boaters report smoother idling, better fuel economy, and fewer breakdowns after install. It's not just a part; it's peace of mind for your time on the water.

Maintenance Mastery for Longevity

Extend pump life with these habits:

• Flush after every saltwater trip to remove salt buildup.
• Inspect annually: Feel for impeller play or housing cracks.
• Use ethanol-free fuel to minimize internal gumming.
• Winterize properly—fog the impeller to prevent dry rot.

Common pitfalls? Over-tightening bolts warps the housing; skipping O-ring lube causes leaks. Get it right, and this pump outlasts expectations.