How do I flush my outboard with fresh water?

Flushing an outboard with fresh water · Symptom: You ran in salt, brackish, or silty water, or the tell-tale is getting weak and you want to keep the cooling passages clean.

Quick answer: Flush with fresh water after every saltwater, brackish, or silty-water trip. If your motor has a built-in flush fitting (Yamaha's flush port or Mercury's Flush Connect), hook up a garden hose with the engine OFF and let water run about 10 to 15 minutes, then screw the fitting back on tight. If it doesn't, use flush muffs over the lower-unit water intakes, turn the water on first, then run the engine in neutral at idle only for about 15 minutes. Take the prop off first and never run it without water.

Why it matters (most to least common reasons)

Salt left inside the cooling passages corrodes the powerhead and exhaust passages
Salt can crystallize in hot passages and form deposits that restrict cooling water (Yamaha notes this above about 170 degrees F)
Sand, silt, and mud wear the water pump impeller and plug passages
A partly blocked cooling system shows up later as a weak tell-tale stream or an overheat alarm

Safe owner checks & fixes

Check your owner's manual first. It says whether your motor has a flush fitting, whether flush muffs are allowed, and whether it has two water inlets (those need a special adapter or a muff that covers both).
Built-in flush fitting: with the engine OFF, unscrew the flush hose connector from the lower cowl, attach a garden hose, and run fresh water for about 10 to 15 minutes. Don't run the engine while using the flush port; that can damage the water pump.
If the boat stays in the water, tilt the motor all the way up first so the fresh water drains through the lower unit and exhaust passages instead of sitting in seawater.
Screw the flush connector back on the fitting and tighten it. A loose or missing cap lets cooling water leak out while running, which can overheat the engine.
Flush muff method (only if your manual allows it): remove the prop, slide the muffs over the water intakes, turn the hose on until water squirts out around the cups, then start the engine in neutral and idle it only, about 15 minutes. Keep the pressure up so water always squirts from the muffs, and watch for a steady tell-tale stream.
Shut the engine off before turning the water off. Then rinse the outside of the motor with a gentle hose stream with the motor lowered straight down, so water doesn't go into the cowl air intake.

Parts / tools

Garden hose with good pressure, flush muffs or a two-inlet flush adapter if your manual calls for one, prop wrench, owner's manual.

Call a marine mechanic when

The tell-tale stream stays weak after flushing and clearing the outlet; the overheat warning comes on during or after flushing; water leaks from the flush fitting even with the cap tight; or you see heavy white salt crust or corrosion under the cowl.

Sources

Values not stated here are model-specific; check your owner's manual.