Exhaust bellows and flappers need to be inspected every so often. This is often a neglected part of engine maintenance. The exhaust heat over time will deteriorate the rubber that the flappers and bellows are made of. When the exhaust flappers go bad, water can get backed up into the cylinder heads and cause the engine cylinders to fill with water ( Hydro-Lock). The internal exhaust bellows can leak water into the bildge and cause CO2 to build up in the engine compartment and other areas.