How to Flush an Outboard Motor

Most outboards rely on open cooling, which is pulling water from underneath the boat and then pushing it up through the gearcase via water passages to the cooling system. 

This is done by utilizing a water pump driven by the drive shaft. And because open cooling systems carry the water the boat is navigating in, salt, sand and debris often get sucked in and start eating away at the impeller and other parts of the water pump. This is why changing the water pump or at least its impeller once every season is recommended. 

But before you dive into the toolbox, the most basic thing you can do to keep your outboard’s cooling system healthy is to flush it after every use. It only takes a few minutes, and it’ll help keep the cooling system clean.

Outboard Engine Flushing Tools 

What tools are needed to flush an outboard depend on the model. Some have a built-in flushing connection, but for those that don’t, there are a couple of options. 

You can use a flush bag, bucket, or large barrel, or a set of flushing muffs, which resemble a set of headphones. You’ll also need a water supply, and nothing beats a garden hose for this task.

NOTE: Some of the images used in this post for illustration purposes are from an engine flushing done in our studio during an oil change to keep the motor from overheating. You can flush an outboard motor without removing it from the boat.

Flushing an Outboard Motor

Regardless of how you deliver the fresh water from the hose to the engine, the idea is the same: push clean water into the gearcase to push dirt and debris out of the system.

Many outboard manufacturers have built in a flush connection into the motor, somewhere under the cowl or on the midsection between the powerhead and the lower unit. Check the owner’s manual for specific instructions on flushing your outboard’s make and model.

How to Use Flush Muffs to Flush an Outboard

With the boat trailered, attach a hose to the muffs and the muffs to the lower unit. The rubber cups should be positioned over the water-cooling inlets on the lower unit. 

Start the water, then the motor, and keep the transmission in neutral, since a spinning propeller would be dangerous. If your outboard has more than one set of water inlets, you may need to tape some up since you want to force all the water through one set of openings.

As the motor idles, you should see water flowing from the telltale or cooling system outlet on the motor. After about 10 minutes, turn off the motor and then the water running to the muffs. When you’re done, drain the water out of the cooling system by making sure the boat is level and the motor is trimmed down. 

How to Use a Flushing Bag or Bucket 

Flushing bags can be made of plastic, canvas, or vinyl, or you can use a bucket or barrel. The idea is to keep the cooling inlets submerged while running the motor and feeding clean water into the bag to maintain the water level. For most flushing bags, you’ll need to remove the propeller first.

Place the flushing bag under the lower unit and trim the lower unit down into the bag. Fill the bag with water and keep the hose running. Next, start the motor and let it idle for about 10 minutes. As it idles, it’ll draw water out of the bag while the running hose replenishes the water level in the bag. 

Turn off the engine, then tilt it up out of the bag or bucket. Reinstall the propeller, then tilt the motor back down to drain out any remaining water.