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Inspecting Exhaust Manifolds And Risers

Keeping a wary eye on raw-water cooled manifolds and risers can prevent the premature death of your engine.

Pop Quiz: The owner of a 25' powerboat hops aboard his boat one day to go for a cruise, but his inboard V-8 engine is slow to start. When it finally does start, he notices a distinct knocking noise which he's never heard before. He continues to run the engine briefly, and then shuts it down to investigate the cause. Eventually he removes the spark plugs and discovers water in the # 2 and #4 cylinders (hmm ...). Later, when the engine is torn down for inspection, it is discovered that the #4 piston connecting rod is bent and the cylinder walls are rusted due to water intrusion. How did water get into the cylinders? (Hint: It's not supposed to be there.)

There are several possibilities, but if you guessed "Act of God," try again. More likely, saltwater passed into the cylinders through a leak in either the raw-water cooled exhaust "manifold" or the "riser." Once water gets inside the cylinders, the result is usually catastrophic engine failure. It can ruin your day, and much of your summer by the time busy mechanics get around to a total engine rebuild (if possible) or replacement. Because the engine is often the single most expensive part of your boat, it makes sense to inspect or replace the risers and manifold periodically before an internal leak occurs, which is more a question of "when" than "if." Once it happens, there is usually little or no warning before the engine is ruined.

Keeping The Water And Gas Separated

Exhaust manifolds and risers are large metal castings that carry hot exhaust gasses away from the engine block on inboard engines. All V-8 engines, for instance, have a separate exhaust manifold along the side of each cylinder bank. The riser, which is shaped like an inverted "U," is sometimes located at the aft end of each manifold (e.g., on Chrysler engines), and sometimes it's centered on top of the manifold (i.e. on MerCruiser engines). Sometimes the riser slopes down from the end of the manifold, if the engine sits high enough above the waterline, in which case it's often called an elbow. The exhaust hose is then attached to the aft end of the riser or elbow.

What makes these cast iron parts unique is that they are a double-walled pipe within another pipe. This arrangement allows hot exhaust gasses in the internal pipe to be surrounded by an external water-filled pipe, called a water jacket, which remains cool enough to touch. At the aft end of the riser, water from the water jacket combines with and cools the hot gasses before continuing out the exhaust overboard discharge. Without the cooling effect of the water, the exhaust gas would overheat the manifold and risers and burn through the exhaust hose in short order.

Keeping the cooling wate