Catalytic Converters on Marine Gas Engines

By Andrew Spaulding

There has been a lot of chat about the new requirement for catalytic converters on marine gas engines. California started the trend by requiring cats on gas engines under 500 horsepower in 2008. The Environmental Protection Agency thought this was a good idea and extended the requirement to the other 49 states for the same engines built after January 1, 2010. Why catalytic converters? Catalytic converters use chemistry to remove most of the hydrocarbons, nitrogen oxides and carbon monoxide from exhaust fumes. The emissions removed are some of the most harmful to our environment and air quality.

So what does this mean to you, the boat owner? Your engine is going to be a bit more complicated with additional sensors and electronics. To make sure the catalytic converter is working properly, oxygen sensors are required in the exhaust stream before and after the catalytic converter to measure the fuel/air mix in the exhaust. The oxygen sensors report to the engine’s electronic controls to adjust the fuel/air mixture as necessary and to deliver an exhaust gas mixture that the catalytic converter can clean up, leading to the fewest harmful exhaust emissions.

How do catalytic converters work? The part of the catalytic converter that does the work is a mesh honeycomb covered with a mix of metals called the wash. The metals in the wash are platinum, palladium and rhodium. As the exhaust gasses pass over the wash, chemical reactions take place and the hydrocarbons, nitrogen oxides and carbon monoxide are turned into water, carbon dioxide and nitrogen. While the resultant carbon dioxide is still a greenhouse gas, it is not poisonous to humans in low concentrations as is carbon monoxide.

In order to operate properly, catalytic converters need to run at high temperatures, which as we know, don’t always mix well with fiberglass and human beings. To address this issue all marine catalytic converters have a water jacket over them to reduce the external temperature…still hot, but the standard for temperature is one at which human skin won’t blister.

What maintenance issues do catalytic converters cause? For the do-it-yourselfer there are a few important things to keep in mind. Some marine grade lubricants contain high levels of phosphorus which will damage the catalytic converter. Be sure that you are using factory recommended lubricants in your catalyzed marine engine. Power Tune Engine Cleaner contains compounds and solvents that are not compatible with catalytic converters. Acetoxy silicone sealants can damage oxygen sensors and catalytic converters, but oxime silicone sealants are safe to use. Leaded fuel and non-approved additives can contaminate catalytic converters.

If you have ANY questions regarding the products that you use and their effects on your catalyzed marine engine, consult your owner’s manual and the manufacturer of the engine. What you used in the past may damage your new engine or void the warranty. If you are having trouble getting an answer, let us know so we can help.