Your Outboard Needs a De-carbon

That probably sounds Greek. So what does it mean when your factory-certified service tech says, "We need to perform a de-carbon due to excessive cylinder leak-down."

What he just referenced is the manner in which compression and leak-down tests measure a combustion cylinder's ability to produce pressure as the piston reaches the top of its stroke and, more specifically, the ability to maintain it. The de-carbon process is a method of removing harmful carbon build-up by injecting a specialized cleaning fluid into the engine while it is running.

Automobile engines generally operate in the range of 2200 RPM at normal highway speeds while 4-stroke outboard engines reach speeds up to 6000 RPM quite frequently. Very different.

An automobile has an oil sump that is primarily air-cooled, a cooling system with thermostats which operate at higher temperatures, and uses oils designed to achieve lower emission levels while also having exhaust systems that operate at thousands of degrees to burn off any excess hydrocarbons in the exhaust gases.

By comparison, 4-stroke outboards have oil sumps and exhaust systems which are immersed in water when the boat is not on plane, creating drastically lower operating temperatures. 4-stroke outboard oil is blended for greater lubricity, necessary because the engines turn faster. Combine this with very high percentages of low-speed operation (which do not allow higher, cleaner operating temperatures to be achieved) and perhaps you will understand how carbon build-up in the combustion chamber occurs much faster than in an automobile.

When carbon builds-up in the combustion chamber, piston rings stick in their grooves and cannot seal tightly, cylinder walls become glazed (which allows exhaust blow-by and excessive crankcase pressure) that in turn causes gaskets to fail. When rings sticks and cylinder walls glaze, unburned fuel gets past the rings and contaminates crankcase oil–a condition called "making oil." Highly detrimental to long engine life.

This problem is easily solved by changing the technique of engine operation (running the boat at different speeds) and performing a "de-carbon" every 300 to 400 hours of operation based on the engine history report.

The 100 hour or annual service is essential for many reasons and the engine diagnostic report records all engine profile information as well as operating RPM (low, medium, and high-range) and the percentage of time operating in each. Collected during successive service visits this information is invaluable when troubleshooting future problems.

Chris Mapp
Coastal Bend Marine
Port O'Connor, TX