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 water and exhaust gasses separated until they exit the riser is crucial. If water finds a 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 "hydro-lock" condition. That happens because water cannot be compressed in the cylinders, so the engine suffers massive and usually 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 travelling at high velocity. They are exposed to saltwater and vibration, and then left idle for long periods while rust and corrosion eat away at the metal and clog water passages. It's a wonder that they last as long as they do. Unfortunately, this kind of loss is usually the result of long term corrosion, which is not covered by insurance.
Reducing The Odds Of Failure
The warning signs of a potential manifold or riser failure are elusive. Hard starting and an unfamiliar knocking sound, as in the claim above, is one way to discover a leak, but by then the damage is usually done and the engine must be completely rebuilt or replaced. When the water-jacket of a manifold or riser becomes clogged with scale and corrosion, a leak is usually close behind because the internal walls are deteriorating. Suspect this if the engine overheats at high speed or under a load, but overheating may also be caused by a damaged impeller, clogged raw-water filter, or restricted raw-water intake screen.
Sometimes it's possible to detect hot spots by comparing the temperature of two risers with your hand; if one seems relatively hot it may indicate that the water passages on that side are clogged. In which case, you may also notice less water exiting the exhaust discharge if each side has an independent exhaust hose (unlike most I/O's). Note that the engine temperature gauge may not indicate overheating when idling at low speed. However, all things being fairly equal on V-block engines, if one side becomes clogged the other is usually not far behind, so the touch test is not very conclusive. A more scientific approach is to hire an engine surveyor who has an infrared pyrometer. This instrument can determine temperature variation in cast-iron exhaust parts much better than your hand, plus you profit from the surveyor's experience.
Another sign of trouble is corrosion at the joint between the manifold and riser, which means the gasket between them has failed and water is seeping out. If water is seeping outside the joint, there's a high probability it will soon leak along the inside joint to the gas-only chamber, if it hasn't already. Don't wait for confirmation on this one.
In fact, the best policy is to not wait for any warning sign at all, but to periodically remove the risers for inspection, and possibly the manifold depending on what you find. How often depends on where the boat is located and how often it's used. Boats operating in Florida may get two or three years out of risers and not much more for manifolds, while those in Maine may last five or six years. It's understandable that there's no primordial urge to do this; after all, your car mechanic never nags you to inspect the risers (probably because car engines don't have risers). Nevertheless, on boats that operate in saltwater, it should be considered standard maintenance. If it helps, remember that the BoatUS Marine Insurance claims files show that most owners had absolutely no warning that they had a problem until it was too late, fun over.
What To Look For
The good news is that risers are not that difficult to remove for those inclined to do it themselves; only four bolts hold it to the manifold and the exhaust hose is held on by a few hose clamps. Once off, you'll be able to see if all the water passages are open or clogged. Take a large flat head screw driver and dig around for corroded metal that's easily removed. If you hit solid metal right away, it's probably okay, but if large chunks flake off, replace it. There should be no evidence of rust in the gas-only chamber on the side that connects to the manifold (expect to see black carbon). If there is rust, water is probably leaking in through a hole in the water-jacket at the upper loop of the riser.
Also look carefully at the metal faces where the manifold and riser join together. Since the walls of these faces are fairly thin to accommodate the water passages, the gaskets tend to deteriorate and develop leaks at the narrowest sections. Sometimes water seepage can be seen outside the joint, as noted above. If the metal face is pitted on either the manifold or riser, replace it or have the surface machined smooth again; otherwise it will probably leak again, perhaps inside the gas-only chamber.
If you decide to replace the risers (always replace both at the same time), you need to decide whether to also replace the manifolds, which are both more costly and a bit more difficult to remove. Any sign of rust in the gas-only chamber indicates a leak, which obviously warrants replacement, but it's more difficult to inspect the interior of a manifold. Risers tend to fail before manifolds because the water is hotter by the time it reaches them, which accelerates corrosion and causes the salt to separate and stick to the walls more readily. Also, the top and aft side of the riser is exposed to both saltwater and air where the two combine, which is a perfect environment for corrosion, so you may get a few more years out of a manifold.
Unfortunately, there is no practical, conclusive method to evaluate the internal condition of manifolds. It can be removed, cleaned, and even pressure checked, but there's no guarantee it won't develop a leak soon after. In the end, you may have to weigh the cost of replacing an aging manifold against the cost of a replacing the engine if it fails.
Another option is to convert your raw-water cooled engine and manifold with a freshwater cooling-system conversion kit. This will increase overall engine life and you won't have to worry about raw-water corrosion in the manifold. Risers, however, are always raw-water cooled, so they will still need to be inspected regularly.
A word of caution: Exhaust hoses disconnected from the riser provide an open path to the outside. They must be plugged and tied securely above the waterline to prevent water from flooding the boat.
Schedule your inspection today - keep in the know with your boat's exhaust manifold and risers.
(credit to boatus)