Maintaining your Diesel Exhaust

The phrase “out of sight, out of mind” is all too true where boats are concerned, and some of the systems that are out of sight on a typical sailboat can really ruin your day—or your season. Take the exhaust mixing elbow, for example—and give yourself a pat on the back for actually knowing what it is.
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The phrase “out of sight, out of mind” is all too true where boats are concerned, and some of the systems that are out of sight on a typical sailboat can really ruin your day—or your season. Take the exhaust mixing elbow, for example—and give yourself a pat on the back for actually knowing what it is. And another for caring about what it does.

What it is, basically, is a well-concealed problem spot on marine diesel engines. What it does is mix the exhaust gases from the engine’s combustion chambers with the seawater used to cool the engine, thereby ensuring that the furnace-hot gases don’t set your boat ablaze on their way out. This is known as a wet-exhaust system and it works well, provided you follow a couple of basic rules: the exhaust must pass through a waterlock muffler located far enough below the mixing elbow that water can’t flow back into the engine, and, if the lowest part of the mixing elbow is below the boat’s waterline, a siphon break must be fitted for the same reason.

A marine diesel has one of two types of mixing elbow, depending on how high in the boat it is installed. A low-rise elbow curves downward from the exhaust manifold. A U-shaped high-rise elbow is usually seen on sailboats, where engines are typically located partly or entirely below the waterline. On most sailboats, the mixing elbow is far enough above both the waterline and the waterlock muffler to not need a siphon break to ensure water can’t flow back into the engine. These are annoying things that tend to clog up and need constant inspection. The waterlock muffler—in which water forms an air lock when the engine stops, preventing it from flowing back toward the mixing elbow and possibly making its way into the engine’s cylinders—is essentially maintenance-free. And as long as the exhaust line is looped above the point at where it exits the hull, there is virtually no danger of water siphoning back into the system.

Such an exhaust system needs little attention (except for those dratted siphon breaks) and will last a very long time—except for the exhaust mixing elbow. I’d never given the high-rise elbow on our 10-year-old Yanmar 2GM20F any thought at all, until a couple of seasons ago I noticed that the engine seemed reluctant to rev out and that the stream of cooling water from the exhaust seemed to have lost its gusto, as if it had some kind of blockage. I also noticed spots of rust bleeding through from the inside of the elbow casting, and telltale bubbles where the exhaust hose connected to the elbow—indicating that gas and water had been leaking out. It was time to act.

Mixing elbows are considered a consumable item, like oil filters and alternator drive belts. I’ve heard varying reports on how long they last. Every five years or 700 hours seems to be an average, but there’s no hard and fast rule on when they need to be replaced. Both the water-cooling inlet and the exhaust passage tend to clog up, the former with scale and salt deposits, the latter with carbon and soot. The corrosive gases eventually eat away at the cast iron, and cracks and pinhole leaks can develop.

Depending on the state of the elbow, it may be possible to remove part of the blockage by soaking the elbow in muriatic acid or taking it to a radiator shop. This is probably a false economy, but it may buy you another year or two. The mixing elbow for our Yanmar cost $130, which averages out at $13 a year. I’ll happily pay that for peace of mind, rather than risk the boat filling up with carbon monoxide or having seawater flow back into the engine.

So I got out the toolbox and set to work. There are three components to our Yanmar’s exhaust mixer: the cast-iron riser that is bolted to the exhaust manifold, the mixing elbow itself and a double-threaded stainless steel pipe that connects the two together. The four bolts connecting the riser to the manifold came out easily enough, but it proved impossible to separate the exhaust hose from the mixing elbow. I had to cut it off, a procedure that involved a sharp knife, a hacksaw and some inventive cursing—the latter because I would now have to replace the hose.

The next step was to separate the three components, a procedure that involved a) twice-daily applications of Kroil for a week b) more cussing when I found I still couldn’t budge it, and c) giving it to a friend with a large workshop and willing helpers. The riser looked fine, with little buildup of hard soot, but the mixing elbow looked horrible, with heavy carbon buildup visible as far as I could see. I couldn’t see into the water passage, but I had no reason to think it looked any better.

As anyone who’s worked on old boats knows only too well, it is much easier to put things on than take things off. The opposing threads on the connector are supposed to draw the two elbows together as it’s tightened, but I found that the mixing elbow was about a quarter-turn adrift of its optimum position after the riser was snugged tight. A healthy glob of high-temperature exhaust sealant was applied to the threads, and the assembly was left overnight to dry. It had set rock hard by the morning.

Reassembly was the easy part. I scraped the remnants of the old gasket off the riser and the exhaust manifold with a razor blade, applied a little anti-seize compound to the four bolts, put the new gasket in place, and bolted on the elbow assembly. Because I’d had to cut the old hose off, I installed a new length of 2in ID Vetus exhaust hose between the mixing elbow and the waterlock muffler, replacing the old hose clamps at the same time.

The four clamps brought the total cost of the mixing elbow replacement up to $270; the elbow and gasket added up to $150 (costs vary a lot, so do your research online and shop wisely), and a 6ft length of hose costs $98 (unfortunately, it’s not sold by the foot, which is all I needed). I should have tried harder to get the hose off the old elbow. Still and all, it was an easy (if rather dirty) project, and the engine is happier for it. It is revving higher and the water flow from its exhaust is again strong.

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