Flushing by the Book

If you haven’t read your engine’s owner’s manual, there’s a good chance you have been flushing your engine wrong.

A few months ago, my neighbor Kat asked if I could help her learn to drive her 31-foot Pro-Line. Her son, J.K., had always done the chauffeuring, but she wanted to take it out herself and learn how to care for the boat when we were done with the trip. It’s powered by twin Honda 225 outboards, and I knew the last thing we would do after a run in the brackish St. Johns River in Jacksonville, Fla., was flush them. So before embarking on our first run, I went to the Honda website (marine.honda.com) and looked up her motors’ owner’s manual. When we were done with her first lesson, I hooked up the garden hose to the flushing port, turned on the water and sat down to wait. “Aren’t you going to start the motor?” Kat said. “J.K. always does.” I told her the owner’s manual says to flush them with the engine off. I could see by her smile that she couldn’t wait to spring this bit of trivia on him the next time they went out.


Flushing an outboard engine is not a one-procedure-fits-all thing. To do it right for your particular model, look it up in the owner’s manual.


In the case of the 2005 Honda BF225 motors I flushed, the manual says to connect a garden hose to the flush port, which is on the upper starboard side of all Honda models more than 50 hp. It includes the cautionary notice that running the engine while flushing with a garden hose connected to the port could damage the engine. The procedure for the Honda BF25 through BF50, however, is completely different. It recommends using the optional flush port on the lower unit. After removing the prop, attach a garden hose and then start the engine. The recommended flushing time for both is 10 minutes.


For direct-injected two-stroke OptiMax outboards, Mercury recommends using a water supply with no more than 45 psi (pounds per square inch) and attaching it to the flush port on the back of the engine, beneath the cowling. Mercury doesn’t give preference to running the engine or not while flushing — three minutes is the minimum either way — but it shouldn’t exceed minimum idle speed if it’s running.

For supercharged four-stroke Verado outboards, Mercury recommends using the fitting on the port side, just below the cowling, and turning the hose on about half strength. When using this port do not run the engine. Know that standard muffs don’t work and could damage the engine, because there is another inlet on the front of the lower unit, in addition to the ones on the side.

The procedure for smaller FourStrokes requires removing the prop, attaching the hose to the fitting beneath the cavitation plate, turning the hose to half strength and running the engine for at least five minutes.

A flush bag is one way to flush an outboard after a day on the water. It needs to be watched the entire time, to ensure the water level stays high enough.

A flush bag is one way to flush an outboard after a day on the water. It needs to be watched the entire time, to ensure the water level stays high enough.


On every model except the DF2.5 portable, there is a built-in freshwater flushing system port in front just under the cowling, and on select other models, there’s another on the port side halfway between the cavitation plate and cowling. The latter has a screw-on cap that needs no tools to remove. Just unscrew it and attach an optional fitting that accepts a garden hose. When using these ports, attach a garden hose and do not run the engine. Suzuki also provides the option to use ear muffs to cover the intakes on both sides of the lower unit. Make sure it fits snugly, then turn the water on first and run the engine at idle. As with outboards of all makes, ensure that water is coming out of the “tell-tale” pilot hole, to verify it is getting proper flow. In both procedures, Suzuki recommends flushing for at least five minutes.


On every current North American Yamaha model of eight hp or more, there is a flush fitting on the starboard side toward the front. And all have the same procedure, which is to hook up a garden hose and run it — with the engine off — for 15 minutes. If the boat is still in the water, tilt the outboard up while flushing. When done, tilt it down to drain. On Yamaha’s YouTube channel is a service video that demonstrates how to use a flush bag, which completely immerses the lower unit in a small pool of water, and muffs. Yamaha recommends removing the prop before proceeding with either. With both of these options, run the engine at no more than 900 rpm for 15 minutes. Always watch the motor during the entire procedure in case the muffs slide or the bag loses water.


Evinrudes have a water port on the back of the lower unit and, according to the manual, it is not necessary to run the engine to give it a good flushing (a bit ambiguous). To use muffs — on the 20 and 30 hp models as well as motors with the M2-type gearcase — it is necessary to use heavy tape to close additional openings, to ensure proper water flow, which makes a great case for just using the port.

On all models that have a port, regardless of the brand, you must replace the cap or reattach the fitting; otherwise, you can cause engine damage, because the water flow will be interrupted. We can’t stress this enough: Read your owner’s manual.