Outboard Flushing The Right Way

It sounds simple, but hang on! There’s a right and a wrong way to perform this essential task

Article by John Tiger

Photo of a man flushing his outboard

Back in the day, flushing an outboard with fresh water was done only one way. A set of "ear muffs" or "flush muffs" was fitted around the engine’s gearcase to cover the water intakes, connected to a garden hose with a good water supply, and the engine was run for five to 10 minutes. But today’s outboards can be flushed using other, sometimes easier methods, without even starting the engine.

Why Flush?

Salt and brackish water is a corrosive killer of the aluminum from which outboards are made, so flushing every time after saltwater use is a must. Left unchecked inside the cooling passages, saltwater will quickly build up and may cause cooling blockages, leading to overheating and, over time, can corrode an engine from inside out. All outboard manufacturers recommend flushing (according to the procedures outlined in the engine owner’s manual) after every use in salt, brackish, dirty or polluted waters. Operating an engine in sandy, silty or muddy fresh water also dictates the need for periodic flushing.

The Old-School Way

Flush muffs are the most common way to flush an outboard; they’re available at most marine stores and online resellers. They’re inexpensive, and easy to use. Connect to a garden hose, fit the muffs over the engine’s water intakes on the sides of the gearcase, turn the water on, start the engine, and let it run. That’s it, with the following precautions:

    Photo of flushing an outboard motor
  • If your engine has additional water intakes that are not being directly fed water via the flushing muffs, they must be sealed off with a bit of duct tape, or overheating may occur.
  • If your engine doesn’t have water intakes on the sides of the gearcase, you’ll need a special type of flushing attachment that covers the front of the gearcase. These can be purchased from aftermarket shops (www.bobsmachine.com).
  • When attaching the muffs, be sure they cover the water inlets completely, and don’t pop or slide off when the water is turned on.
  • Be vigilant, and don’t leave the engine while flushing. Watch the engine’s “tell-tale” overboard water indicator to ensure that the engine is pumping water.
  • The engine should be kept in neutral and not run above a fast idle speed (1,000 rpm maximum).

Built-In-Flushing Attachments

Built-in garden hose attachments are a standard part of many outboards manufactured in the past decade or so. These attachments make flushing easier, because the engine usually doesn’t need to be running (and in some cases, should not be) to accomplish the flushing procedure. Simply check to see if your engine has this attachment (look in the owner’s manual), find the connection point, hook up a garden hose, turn on the water, and let it flow for 10 minutes. This is handy if you can’t start the engine (dead battery, for example) or if your neighbors would balk at your engine’s noise. Some caveats about using a flushing connection:

  • Be careful not to cross-thread the connection. Many are plastic, and can be ruined quickly if threaded incorrectly with the brass fitting of a garden hose.
  • Pay attention to the manufacturer’s recommended procedure. As follows.

Flushing Bags

Flushing bags are soft-sided heavy plastic/vinyl bags with a support framework, designed to be fitted up and around the lower end of an engine and fully enclose it. A garden hose is attached to fill it and keep water circulating; the engine is then started and run much the same as with a flush muff attachment. Flush bags allow for engine-on flushing without the loud noise of the engine’s open exhaust. Online, Google "outboard flushing bag," and several options pop up. As with other flushing devices, caution must be exercised:

  • The bag must fit pretty snugly around the engine’s midsection, and be tall enough to reach well above the water intakes.
  • The propeller should be removed before installing the bag.
  • As with a flush-muff attachment, engine speed should be kept to a fast idle, no more than 1,000 rpm. The engine should be kept in neutral.

Manufacturer’s Recommendations

Most outboard manufacturers’ flushing recommendations are similar. All of them make a statement about ensuring that all water is drained from the engine after flushing. This is especially important in freezing climates, so that there is no water left inside the engine that could then freeze and cause damage. However, there are some differences, so here they are, straight from the manufacturers themselves:

Evinrude (BRP) For flush-muff attachments, temporarily cover the auxiliary water inlets with heavy tape on their 15- to 30-hp models and high-performance Lightning M2-type gearcases (remember to remove the tape afterward). Water pressure should be between 20- and 40-psi. Flush for at least five minutes at an idle speed, with the engine in a vertical position. When using the flushing port, it is not necessary to run the engine. Keep the engine vertical after flushing so that all water drains from the powerhead.

Honda Honda’s flushing procedure varies by engine size and model. In all cases, the engine must be in the vertical position during flushing and after, to drain the water from the engine. Smaller engines (2- to 20-hp): Flushing in a small container (such as a flushing bag or small bucket or can) is acceptable.

For engines 5-hp and up, an optional flushing attachment (Honda part #06190-ZV1-860) is available that allows flushing the engine through a port on the side of the gearcase. The prop must be removed and the engine run for about 5 minutes with either procedure. Either method is acceptable.

Midsized engines (25- to 50-hp): Use only the factory "WASH" port located in the lower port/left side of the gearcase. Again, the engine must run, with prop removed, for about 5 minutes. Honda warns that if water pressure is low, tape should be used to close off the engine’s water intakes on each side of the gearcase.

Larger engines (60- to 250-hp): Honda’s only recommended flushing procedure is to use the factory flushing port connected to a garden hose with the engine not running.

Mercury Mercury’s procedure varies by engine size and series. Smaller and midsize four-stroke outboards: The engine must be warmed up to open the thermostat and circulate water throughout the engine’s cooling passages completely. The engine can be vertical or tilted. Remove the prop, and run in neutral at no more than an idle speed for at least 5 minutes. The water supply should not be opened more than halfway to regulate water pressure.

Photo of a Mercury outboard

Optimax two-stroke outboards: Remove the prop, attach the muffs so that the rubber cups fit tightly over the water intakes, then adjust water flow so that some water leaks out around the cups. Start the engine and run at idle speed in neutral for 3 to 5 minutes. For flushing without muffs, Mercury offers a gardenhose adapter that connects to a port in the lower engine cowl; this port is accessed by removing a dust cover. Mercury makes special note of using this adapter to drain the cooling water from the engine by disconnecting it from the water-supply hose, then tilting the engine to allow all the water to drain out. This is especially important in freezing climates. Water pressure no higher than 45-psi should be used. The engine can be running or not when flushing, and flushing should be for at least 3 minutes. Mercury notes that this procedure should be used when flushing the engine after use in salty or dirty water, and also as a part of preparing the outboard for storage.

Verado four-stroke outboards: No description of using flush muffs is shown in the manual, only the engine’s hose-adapted fitting. The description of the procedure is short, noting only that the engine should be off and can be either tilted or vertical.

Suzuki The preferred method is to flush with a flush-muff attachment or built-in port. With flush muffs, run the engine only at idle speed in neutral for a few minutes while monitoring it at all times for proper water flow out of the tell-tale outlet. There are two possible flushing ports — one on the port side of the engine midsection, the other on the front side of the lower cowl. The engine can be running or stopped; flush for about five minutes, and ensure the engine drains completely. When not running, the engine can be flushed in the tilted position, but must be returned to the vertical position to drain afterward.

Tohatsu/Nissan For smaller engines (2- to 6-hp), use either the optional flushing plug or a small container such as a garbage can or tub filled with fresh water. For larger engines, use the flush port or a set of flushing muffs. Water pressure should be set to one-half or more to ensure adequate flow to the intakes. The engine should be run for 5-10 minutes.

Photo of a Yamaha outboard

Yamaha Yamaha gives three choices — with a flush bag, muffs, or hose-port connector – and says all three methods work equally well. Bag and Muffs: Engine should be vertical, run no more than 800-900 rpm in neutral for 15 minutes with the prop removed.

Be sure the engine is receiving cooling water (by checking the overboard indicator). If your outboard has more than one set of cooling inlets, a flush bag should be used. Flushing Port: Engine should not be running; it can be tilted or vertical. Prop should be removed. Flush for 10-15 minutes.

John Tiger owned his first outboard at age 7; since then he has owned more than 60 boats and outboards. He started outboard racing at 14 and is still active, building racing engines and rigging performance boat in his ship in upstate New York. Seaworthy is the BoatUS Marine Insurance publication dedicated to helping members avoid injury and boat damage due to accidents and storms. It’s published four times annually. For more great articles visit www.BoatUS.com/Seaworthy