Why Does the Outboard Coolant Stream Sometimes Steam?

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A BoatTEST member recently asked about the stream of water coming out from the rear of this engine. “Why does the water streaming out of outboard steam when I slowdown and come to a stop, but when I start again, it goes away?”

Bill Grannis, a well-known industry expert on outboard engines answers–

The steam you notice is a normal occurrence on most outboard motors. The exhaust gases exiting the engine block may reach the 1,200-1,500-degrees F range when under heavy load. They are directed down an exhaust chamber inside the midsection and out the propeller hub. Some of the cooling water from the engine is directed to quiet and cool the exhaust gases as well as surrounding the exhaust chamber, further silencing the motor.

At higher speeds the exhaust chamber becomes very hot internally and the cooling water injected into it turns to steam and is expelled through the propeller hub while the boat is underway. When you slow down by pulling the throttle back, the pressure and the volume of exhaust is greatly reduced, yet high temperatures remain inside the chamber and the steam then exits through the idle relief port.

The whitish gray vapor that you see coming out of the back of the motor is perfectly normal. As the motor idles, the exhaust gas temperatures subside and the visible steam practically disappears. In cooler humid weather, you may notice it more, just like out of the exhaust pipe of your vehicle on a cold day.

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