Will a Leaking Exhaust Manifold Only Make Noise When Cold?

Operating a boat with a leaking exhaust manifold is wrong on many levels. If the leaky manifold is hissing like a tea kettle, it may be caused by one of two things: a damaged exhaust manifold water jacket or a funky exhaust riser. The leaky exhaust manifold can kill you or your passengers with carbon dioxide; the funky exhaust riser can cause your engine to hemorrhage internally. The problem is readily repaired with a new exhaust manifold, riser, hose and gaskets.

  1. Exhaust Manifold

    • Your boat engine's exhaust manifold is a single part, but like many parts, it's made up of more than one component. The first component is the exhaust manifold itself -- the exhaust chambers that collect the exhaust gases. The exhaust gases are as hot as the inside of the combustion chamber. They are sucked into the manifold's exhaust chamber immediately after the exhaust valve opens -- as is the case in a four-stroke engine -- or is exposed by the movement of the piston in a two-stroke engine. These gases are hot enough to melt or deform the exhaust manifold and would -- except for the manifold's second component.

    Keeping It Cool

    • The exhaust chambers, made of iron, would melt if it were not for the manifold's second component: a water jacket that surrounds the exhaust chambers and associated piping like a skin. Also made of cast iron, this water jacket is filled with water drawn from the water in which you are bobbing around.

      An outboard has water inlets on its lower unit that channel the water into the water pump that in turn circulates the "raw" water to provide cooling throughout the outboard, including the exhaust manifold.

      An inboard, or inboard/outboard, usually cools by a closed cooling system, and a specialized radiator called a "keel cooler" is located below water to exchange the engine heat with the water surrounding the boat. Even with this closed system, the exhaust manifold is cooled by a separate raw water pump and raw water cooling system. This system sucks cooling water from around the boat. It then circulates it only once through the manifold water jacket and is discharged from the boat.

    Hissing

    • Hissing indicates the escape of steam. Water doesn't hiss as it escapes, but steam does. Charles' Law says that as water is heated above its boiling point, its volume expands. If its volume expands in a closed system, you won't hear a hiss. If it finds an opening, it announces its presence with a hiss, like that of a hissing tea kettle.

    Hissing When Cold

    • Since the exhaust manifold cooling system is a closed system, a hissing sound means there's a gap somewhere in the otherwise sealed system of the exhaust manifold. The hiss means the manifold's water jacket almost certainly has a crack in it. This crack may or may not be associated with an exhaust leak from the manifold. The key to diagnosing the problem is that there are only two ways water can gain access to the manifold's heat. If the water jacket is leaking, the hissing continues as the engine heats up.

    Hissing When Not Cold

    • If the engine's leaky manifold stops hissing when it warms up, it's likely small amounts of water disturbed by the boat's movement are back-flowing through the exhaust hose into the riser then into the manifold. It loudly hisses as soon as the engine starts, but it may stop as soon as the engine warms up. This happens because the gasket between the riser is damaged, deformed or absent, or because the riser is rusted out and allowing ambient water to directly gain access to the manifold. This is problematic. If the water flows back into the manifold, it is sucked into the combustion chamber before the exhaust gas is expelled. Enough of this cold water flowing through the exhaust valve cracks the engine block. This event usually is preceded with a warning: The riser will shake uncontrollably and clang like a dropped box full of scrap metal.