Engine Colonoscopy
Examining your manifolds and risers now might save you heartache in the future.
You know how, when you visit your doctor, s/he always has a bunch of tests you ought to get, just to make sure nothing serious is growing in your innards? Things like a bone-density test, a screening for lung cancer, various blood screenings, maybe an MRI for one reason or another (it’s like being buried alive, but noisier) and, of course, the dreaded but always recommended colonoscopy. None of these is pleasant, and each can be costly, but let’s not kid ourselves: Getting really sick is a lot worse than having some medical technician draw blood or stick an endoscope up your backside.
The same thing is true of your engine: It can develop problems that will shorten its life, ruin your boating enjoyment and thin out your bank account. But you can keep your motor happy and healthy for years by playing doctor, giving it regular and thorough examinations to nip in the bud serious, possibly fatal, maladies. One of the most important—maybe THE most important—screenings you can do is inspecting the exhaust system for leaks and corrosion, internal and external. Checking exhaust manifolds and risers is as important to engines as a colonoscopy is to you and me. And it’s a lot less humiliating.
“Manifold and riser failure is a close second to improper winterization as a cause of engine failure,” says “Jim the Boat Guy” Valiante, a mobile marine mechanic based not far from the Power & Motoryacht offices in eastern Connecticut. The Boat Guy is New England’s largest installer of re-manufactured long-block marine engines, thanks in part to folks ignoring their exhaust plumbing. The manufacturer’s recommended service interval is seven years, says Valiante; that’s OK in freshwater, but in saltwater, he recommends servicing every 5 years: “I emphasize to my customers the importance of being proactive rather than reactive. Waiting for failure can lead to severe engine damage, but if caught early, repairs can often be made.” If you’re not replacing manifolds and/or risers on a set schedule, stay alert for signs of failure, he advises. For example, overheating when the engine’s under load can be caused by rust and scale impeding the flow of cooling water in the manifold. Watch for fluid leaks, which can indicate a failed gasket. It’s easier to spot leaks—whether water, oil or fuel—if the engine is clean, so keep it that way.
What Are Risers?
What exactly are manifolds and risers, and where do they live? Both are components of the engine’s exhaust system, ducting waste gas from fuel combustion away from the engine and out of the boat. The exhaust manifold is bolted to the side of the engine; the pistons push the waste gas out of the cylinders through the exhaust valves into the manifold; from there the gas travels through the riser/elbow into the exhaust plumbing. Each bank of cylinders has an exhaust manifold, so there are two on a V-configuration engine.
The manifold is water-cooled, either by sea water on a raw-water-cooled engine or anti-corrosive coolant on a closed cooling system—what most folks call “freshwater cooling.” Freshwater-cooled manifolds will last a long time, often as long as the engine itself, as will raw-water-cooled manifolds on boats used in fresh water. But exhaust manifolds are usually made of cast iron, so engines cooled by raw saltwater, if not flushed out regularly, will eventually clog with rust and scale and cause overheating issues. Eventually they can corrode enough internally to let cooling water leak into the exhaust passages and then into the cylinders through open exhaust valves when the engine’s shut down. Water in the cylinders is not a good thing: Small amounts will make the engine hard or impossible to start, lose power and run rough. Larger amounts will kill the engine when the piston tries to compress the water in the combustion chamber: Water isn’t compressible, so something has to give, and that’s a piston, or connecting rod or something else that’s fatal to the motor.
The design and components of exhaust systems vary—the height of the engine relative to the waterline is the primary determinant—but the basics of all wet exhausts are the same. (Dry exhausts are simpler and more reliable, common on commercial vessels but rarely seen on pleasure boats.) The exhaust manifold has either a riser, which resembles an under-sink P-trap turned upside-down, or a down-angled exhaust elbow bolted to it. Risers are also usually cast iron, and water-jacketed to keep their surface cool; elbows can be cast iron or stainless steel, and are sometimes wrapped with insulation. (On diesels, there’s often a turbocharger between the manifold and riser, but let’s not complicate things.)
Risers and elbows are always cooled by raw water, either the same water that cools the manifold in raw-water-cooled engines, or water coming from the heat exchanger in closed-cooling systems. As it leaves the riser/elbow, the cooling water is injected into the exhaust plumbing where it mixes with and cools the hot exhaust gas in such a way that no water will flow back towards the exhaust manifold. After passing through exhaust hoses and mufflers, the gas/water mix flows into the sea.
Check Inside
Risers are the Achilles’ heel of any wet exhaust system, more so than manifolds. The hot exhaust gas combining with saltwater promotes not only corrosion, but also scale accumulation inside the riser. Eventually this will restrict water flow and lead to overheating of the exhaust. A pyrometer in the riser will warn you of this, and also alert you right away if there’s a blockage anywhere in the raw-water cooling—it shows up first in the exhaust. Think about installing one, or at least an overheat alarm, in the riser; it might save your engine.
If the water jacket corrodes away inside the riser, cooling water can leak into the exhaust far enough upstream that it can find its way back into the manifold, and then into the engine. How long does it take for this to happen? If you’re lucky, you can get 5 years from a riser, but your mileage may vary. Diesel-exhaust gas is less corrosive than gasoline exhaust, so components tend to last longer. Power and Motoryacht’s original engine guru, Richard Thiel, suggested removing the rubber bellows or exhaust hose from the riser every year and checking for signs of rust, water seepage and other nasties. That’s maybe more often than necessary, but certainly won’t hurt.
Inspecting the riser is a maintenance chore you can do yourself with just a screwdriver and a flashlight. Expect to find rust on the exhaust pipe under the hose; cast iron will rust around saltwater almost while you watch. Clean out the water passages around the exhaust, where the cooling water sprays in. There should be no trace of water inside the exhaust—just black soot. And always—ALWAYS!—replace the exhaust hose immediately after checking the riser; if you don’t, and the hose falls into the bilge, water can run into the boat through the exhaust port in the transom. I’ve seen a very nice lobster boat awash to the gunwales because of this, so it’s not just something we writers make up to scare you. A properly designed exhaust system should have vented loops to prevent water siphoning back into the boat, but not all do, so be safe, not sorry: Reconnect the exhaust hose.
Flushing the raw-water-cooling plumbing at least once a season is a good idea, and will make the components live longer. Flushing it more often is even better. Use a descaling solution, not just water—Salt-Away or Barnacle Buster or something similar. Some raw-water-cooled engines have fittings installed just for this purpose, same as outboard motors do. If you have flush fittings, use them. Finally, check the operating temperature of all your engine components periodically with an infrared thermometer. Hot spots can indicate internal problems even if things look good on the outside.
Do It Yourself?
A garden-variety cast-iron riser kit for a 7.4-liter gasoline V-8 engine costs around $500; manifold and risers, about $800, with parts for both sides of the engine. Of course, you can spend a lot more—maybe you want stainless steel, for example. Aluminum manifolds and stainless risers are a common upgrade, and popular with the go-fast crowd; you might be looking at a couple of grand for a set. But it’s still cheaper than a new engine. (What’s cooler than a high-performance boat with a couple, or maybe three, fire-breathing V-8s pumping exhaust out the transom through stainless-steel pipes?)
Risers and manifolds are simply bolted into place, and should be removable with a socket wrench and average mechanical skills. There are gaskets that must be replaced, too; they can sometimes be installed upside down or backwards, so take a picture of the old ones before you scrape them off. But how often do things go smoothly on an engine that’s been run in saltwater for years, maybe without much maintenance? Jim Valiante said, “Manifolds and risers are heavy and must be drained beforehand to prevent water from leaking into the cylinders. The biggest challenge often lies in the condition of the bolts. In rusty or wet environments, we sometimes have to cut the manifold to remove stubborn bolts, and bolts without heads turn it into a much more complicated job.”
Valiante’s advice is to follow the service interval schedule, replace components as needed and then reset the maintenance timeline. Keep your manifolds and risers intact, and you’ll go a long way to prolonging the life of your engine.
Now, about that colonoscopy…
Have a Look Inside
Most mechanics look inside the risers with a flashlight and poke around with a screwdriver, but it’s more fun to get technical: For a few bucks, buy an industrial endoscope—some folks call them “snake cameras.” I’ve found models that connect to your cell phone as cheap as $20 online. A better one, with its own display, a waterproof probe and the ability to record still images, will cost as little as a hundred bucks, although you can spend a lot more.
A snake cam carries its own LED light, and lets you peer into your plumbing as far as you can push it. Removing the exhaust hose lets you insert the endoscope, which is far better than looking with a flashlight. There are removable plugs in both the riser and manifold, primarily for draining them during winterization. Unscrew the plugs and use the endoscope to check out the inside—it’s not as thorough as removing the riser, but can help: At least you’ll know if the innards are full of rust or clean as a whistle. Once you own the camera, you’ll find lots of uses for it, both onboard and at home.
A snake camera is a lot like the endoscope doctors use to perform a colonoscopy. However, I’m not suggesting you DIY any medical procedures. Neither I nor Power and Motoryacht will take responsibility for your perforated colon. See the doc for that.
