Skills | Techniques & Best Practices


Outboard Flushing,
The Right Way

By John Tiger

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

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 gear cases (remember to remove the tape afterward). Water pressure should be between 20 and 40 psi. (You can buy a pressure gauge attachment at a hardware store to monitor 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'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, 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 gear case. The prop must be removed and the engine run for about five minutes with either procedure. Either method is acceptable.

Midsize engines (25 to 50 hp): Use only the factory "WASH" port located in the lower port/left side of the gear case. Again, the engine must run, with prop removed, for about five 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 gear case.

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's procedure varies by engine size and series. Smaller and midsize four-stroke outboards use the flush port at the rear of the engine. 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 five minutes. The water supply should not be opened more than halfway to regulate water pressure.

Optimax two-stroke outboards: Remove the prop, attach flush 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 three to five minutes. For flushing without muffs, Mercury offers a garden-hose 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 three 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-adapter fitting. The description of the procedure is short, noting only that the engine should be off and can be either tilted or vertical.


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.


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 five to 10 minutes.


Yamaha gives three choices — with a flush bag, muffs, or hose-port connector Ñ 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. End of story marker

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 boats in his shop in upstate New York.

Published: August/September 2013

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