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Inspecting Exhaust Manifolds & Risers

Manifolds and risers are some of the less-discussed "consumables" on your boat. But when they corrode internally, they often destroy the engine.

Nearly every inboard and inboard/outboard (I/O) engine has exhaust manifolds and risers. But ask most people about the life span of these essential components, and you're likely to get responses ranging from "a few years" to "forever."

Ponder for a moment that something made of iron on your boat is exposed to hot, acidic exhaust gas and seawater, and you begin to realize that even "a few years" may be too long. In reality, boats operating in Florida may get only two or three years out of risers and not much more from manifolds, while those in Maine may get five or six years. Boats in freshwater areas can get a few years more — maybe.

It's the "maybe" part that's a problem because manifolds and risers don't have an expiration date or a warning light to tell you when they're failing. And when they fail, they can destroy the engine. Damage to gas engines is far more common, so that's the focus of this article.

What They Do

Exhaust manifolds and risers are large metal castings that carry hot exhaust gases away from the engine on inboard engines. All V-8 and V-6 engines, for instance, have a separate exhaust manifold along the side of each cylinder bank. The riser, which is often shaped like an inverted U, is located at the aft end or centered on top of the manifold. 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're pipes within another pipe. This double-wall arrangement allows hot exhaust gases in the internal pipe to be surrounded by water in the external pipe, called a water jacket, which should remain cool enough to touch. At the aft end of the riser, water from the water jacket combines with and cools the hot exhaust gases before continuing out the exhaust overboard discharge. Without the cooling effect of the water, the exhaust gas would overheat the manifold and risers, then burn through the exhaust hose in short order.

Corrosion between the water and gas passages may let water into the engine, which can destroy it. (Photo: BoatUS Insurance Files)

Keeping the water and exhaust gases separated until they exit the riser is crucial. If water finds its way into the gas-only chamber before the end of the riser because of a leak in the water jacket, it can seep into the cylinders when the engine is at rest and either seize the pistons with rust or create a "hydrolock" condition. This happens because water cannot be compressed in the cylinders, so the engine suffers massive and sometimes irreparable damage when you try to start it. Both result in the premature death of your engine.

Why do these heavy iron castings fail? Manifolds and risers live in a harsh environment. They must endure very hot gas containing corrosive acids traveling at high velocity. They are exposed to saltwater and vibration, then left idle for long periods while rust and corrosion eat away at the metal and clog water passages. It's a wonder they last as long as they do.

Reducing The Odds Of Failure

The warning signs of a potential manifold or riser failure are elusive. Hard starting and an unfamiliar knocking sound on starting are two signs of a leak, but by then th