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Another item in the preventative maintenance category that we recently completed is fresh-water flushing our Yamaha outboard. Believe it or not, both the engine’s manual and the video below recommend flushing the outboard after every use in salt water. Obviously that is not going to occur. I do think that flushing it every so often should still help to keep the cooling system clear.

When flushing ZTC’s 9.9 HP Yamaha we used clamp-on muffs that attach to a garden hose and fit over the cooling water intake on the engine’s lower unit. After ensuring that the water is flowing, the engine is started and run for a while. If it’s working properly, you’ll see the tell tail “pee stream” exiting just under the engine. If you don’t see that, I suspect you’d better stop the engine quickly and figure out what’s wrong!

To flush our new 40 HP 4-stroke I initially thought that I’d have to use the same method, and even went so far as to purchase a cheap set of muffs for the job. I later learned that there is a built in flushing connector just under the engine which is designed to screw directly onto a garden hose. Note that when using this method the engine is not turned on! The water is just left to flow through the cooling system on its own.

As it’s recommended to leave the hose on for 10-15 minutes, completing this procedure uses a fair amount of water. While perhaps not all that much of an issue if you’re hooked up to a dock supply, it’s pretty significant when you’re taking the water from your vessel’s tanks. Good thing we have a hi-cap watermaker!

You can see the white garden hose attached to the engine, and the water flowing out of the engine.


  1. Hello Mike,

    As an ex-owner of a sea-going RIB with two 40hp outboards, the recommendation I was told initially was also to flush the engines after each day of use. This is a bit of a pain, even with the clamp-on muffs. However, I have subsequently heard (and this may also be anecdotal) that a flush is only really required when you are not going to use the boat for a while (or at the end of a week or two weeks or so). Apparently, regular usage (even in saltwater) flush the system adequately, the problem, as I understand, is when the boat is NOT used, that the saltwater in the system starts to crystallize.

    All the best,

    • I suspect that you’re basically right. That said, the 9.9s on ZTC were used all the time. In spit of that, the cooling system is still subject to build up of salt deposits. My hope is that flushing, even if done intermittently, will help alleviate this.

  2. Mike,

    At the end of the season, we flush our 15 hp Merc with a 5 gallon bucket of water filled with Barnacle Buster. It removes a lot of the salt deposits that are in the internal cooling passages. We do the same thing with the heat exchangers on our main engines. I like your built in flush connector!


  3. When the engine sits for a period of time that allows the salt to crystallize flushing with plain old fresh water will not dissolve the salt mineral deposits and flush the system.

    Because of engine heat the crystallization can take place in just a few hours after the engine is run in salt water, hence the manufacturers recommendation for flushing after each use. Each subsequent use of the engine will then add to the buildup of salt and minerals within the engine.

    Failure to flush the engine will result in clogged water passages and inoperative thermostats causing engine overheats and the resultant damage shortening the life expectancy of your motor.

    There is a product “Salt-Away” that is designed to dissolve the salt although it is usually necessary to run the motor for a lengthy time in a tank containing the water/salt-away solution to fully clean out the passages.

    So if your going to flush do it every time or your wasting your time and your water.

  4. Hey Mike, I think I read somewhere that it is not good to force water thru the engine, that the engine should pull the water through. Glad you guys are doing well with your charters.

  5. Do we have any evidence that the calcium crystallization takes place after operation rather than during operation? In fact,I have have some pretty strong arguments that the formation is ONLY related to engine hours and Ca ion concentration, and that flushing will not prevent hard deposits. It will remove some loose deposits and dirt–and these things do matter, since living things and sediment in the passages are bad and can be synergistic to scale growth–but don’t pretend frequent flushing has can prevent hard scale formation.

    a. Scale is formed when water is heated above it’s scaling point (a temperature where the available Ca becomes insoluble). If formation were not temperature dependent, metal parts would become scale-covered just sitting in the water, which they do not. Scale formation is effectively instantaneous.

    b. Scale is also formed when the pH changes. I have seen terrible scaling problems in water treatment plants at the point of caustic injection; valves can become inoperable with hours if not exercised and 4-inch pipe choked in a few months. However, the problem is ONLY during operation and stops when the flow stops. Flushing with fresh water does nothing.

    c. Scaling in diesel engines. There is even an ASTM test for this one. It is all about heat flux and additives that inhibit scale formation, but again, ALL of the scale forms when there is heat and flow, and none when just sitting. Again, it is the extreme temperatures of cylinder liners driving the coolant mix past the scaling point.

    d. Cooling towers. Again, it is about temperatures. They do not scale in the winter, when there is no load, only when heat drives the equation.

    Certainly you can flush, but honestly, you’ve got many better things to do. Barnacle buster or CLR a few times a year will help; the buffered acid will dissolve some of the calcium with limited damage to the aluminum. But I wouldn’t loose sleep over it. The biggest single factor is changing impellers and maintaining good flow; if the water does not get hot it won’t scale. And always allow a short cool-down after running hard; a lot can happen in a few minutes if the cylinder head is put away hot. Flushing would only help if applied before the engine shut down, because the scaling effect is nearly instantaneous; within 1 minute all of the Ca that is going to deposit has and the water is no longer saturated with Ca.

    I’ve done a lot of time with this subject around refineries and diesel engines, and cleaned a lot of choked pipes… but I won’t waste time flushing outboard engines because it isn’t going to matter. Urban legend where the chemistry and math don’t work, like running carbs empty for anything other than seasonal storage.

  6. Mike – read your story from the beginning…just as a heads up…you can also try sugar free kool aid/bug juice, it is a mild acid and is easier to store. We used it to clean our brass fire hose nozzles in the navy.

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