Marine engine exhaust manifolds and risers must be inspected and replaced regularly or they can fail and cause catastrophic damage to the power plant. Risers and manifolds are the part of the engine that allows exhaust gases and cooling water to be discharged from the system during operation.
Manifolds and risers are water-jacketed, which means the water chamber is separated from the gas section by a dividing wall. Over time, hot exhaust gases and raw cooling water, especially salt water, corrode the chamber allowing the water to enter the dry side of the part. When this happens, the water can fill the engine, causing all kinds of damage that can put the engine out of commission permanently. The failure often occurs when the engine is running, which means the misdirected water enters the cylinders when the pistons are moving. Because water does not compress, the connecting rods, push rods, and engine block end up distorting and breaking. Sometimes the damage is so severe that the block is not even suitable as a core for rebuilding. To prevent damage of this magnitude, many manufacturers recommend replacing risers every three years and manifolds every five years. Although changing these parts this often may be may be questionable, inspecting them often is not.
One telltale sign of impending failure is rust around the manifold to riser joint. This joint will often leak during operation, because the water passages are partially obstructed and cannot handle the volume of water that is pumped through the engine. Another sign of a failing riser is an engine that overheats. Many boat owners spend a lot of time and money replacing water pumps, exchangers, and thermostats only to find that their engines still overheat. A competent marine repair facility can pressure test manifolds and risers to check for leaks. Visual inspection is also valuable.
If you take good care of your manifolds and risers, you’ll save money, and maybe even your boat, in the long run.