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White smoke from the exhaust of any internal combustion engine is cause for concern. In an outboard, though, it may mean something other than a tow to shore and an expensive repair. White smoke may mean that water has entered one or more of the outboard's cylinders -- which can cause the engine to suffer irreparable damage -- but it also may mean that the water jacket on the exhaust manifold is leaking. A single test can differentiate between the two problems.
Items you will need
Outboard test tub
Spark plug wrench
Observe the oil pressure and the temperature of the engine. If the oil pressure drops precipitously, shut the engine down and request a tow, using your marine VHF radio. If the oil pressure and temperature remain within the normal operating range, reduce your speed and proceed to the nearest dock or marina by the shortest route possible.
Take note of how the outboard performs when returning to the dock or marina. If it runs poorly -- as if it's starved for fuel -- there may be a small leak around a seal or gasket. Cylinder temperatures will be higher than normal, but oil pressure will remain stable.
When you finally return to your shop, mount your outboard in a test tub, or connect a water source, such as a garden hose, to the outboard. Turn on the water and start the engine. Run it until it reaches its normal temperature. Shut the engine down using the emergency shutoff -- pull the lanyard clip from the throttle.
Take the spark plugs out of the engine, using a spark plug wrench. As you remove the plugs, write the cylinder number -- the top cylinder on the starboard side is "No. 1" -- on a piece of tape and wrap the tape around the plug's ceramic insulator. Thread a compression gauge into the No. 1 cylinder.
Crank the engine over three times. Record the compression each time you crank the engine. Zero the compression gauge after you record the pressure. Move the compression gauge to the next cylinder. Repeat this process for all cylinders. When compared, the highest reading from each cylinder should be within 10 percent of the readings of every other cylinder. If not, the engine requires an overhaul.
Replace the plugs in the engine, if the compression test is satisfactory. Start the engine and allow it to warm up. After the engine is warm, pick up a few drops of water with your hand and drop them onto the engine's exhaust riser. If the drops evaporate, do not touch the riser; the exhaust manifold requires replacement. If the drops do not evaporate, touch the riser with your hand. If the riser is warm to the touch, the water passages in the manifold are clogged. Replace the exhaust manifold.
- Evinrude Repair Manual -- 2.5- to 250-Horsepower Models, 2002-2007; Seloc Marine
- Mercury Outboards Repair Manual -- 2.5- to 250-Horsepower, All Engines 2001 - 2009; Seloc Marine
Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.