Exhaust manifolds last forever, don’t they? Even if they do fail, it isn’t a major problem, right? These are common reactions when people are asked about their boat’s manifold. Unfortunately, exhaust manifolds are important, and ignoring them can potentially lead to expensive problems, perhaps an engine rebuild. There is an additional hassle-manifolds are normally damaged by corrosion, so they’re not covered by your insurance policy.


All of this makes the outlook seem rather bleak. It’s not as bad as it seems, though. All that’s needed is a change in attitude. Rather than seeing it as a “sealed for life” component, view a manifold as a service item to be replaced at regular intervals. If you do this, major problems can be avoided.


How long will a manifold last? Obviously the way you use your boat will be a factor, as will the type of water it’s on. Saltwater boats are going to see a shorter manifold life when compared to their freshwater counterparts. Most experts suggest that a manifold will have a life expectancy of six to eight years. However, heavy use in saltwater can see this drop to as low as three years, while lightly used freshwater boats can get up to 20 years out of a manifold. (We are assuming that your manifold is raw water cooled. While it is the exception to the rule, some engines have a manifold cooled by the engine’s fresh water/antifreeze coolant mixture. These obviously usually last much longer.) One thing is for certain, the older your manifold gets, the more likely it is to fail. This is clearly shown in the chart below which is for illustrative purposes.

Years in Service              Probability of Failure



The manifold is a complex metal casting, actually a pipe within a pipe. It collects the extremely hot exhaust gasses directly from the engine into an inner “pipe” or long chamber which is usually cooled by raw water running through an outer casing. From here the hot exhaust gases and water are fed to the riser where the gases and water combine to continue their trip overboard. Without the cooling effect of the water, the hot gas would burn through a hose or thin wall pipe very quickly. Keeping the water and gases separate in the manifold is critical. If water finds its way into the gas-only section, it can enter the engine cylinders and wreak havoc with the internal engine parts.

Manifolds and risers live in an incredibly harsh environment. They endure very hot corrosive gases slamming into the manifold at high velocity. The water jacket portion of a manifold is intermittently exposed to hot saltwater and moist air, the perfect conditions for corrosion. All the time they’re vibrating madly during running time and left idle for long periods, allowing rust to eat away at the metal. That they last as long as they do is impressive.


Before your manifold fails, you may get warning signals. Needless to say, you should take notice immediately. If your engine is difficult to start, produces white smoke, or runs roughly, water in the cylinders may be the culprit. If you ignore the situation, hydrolock may occur. This is when sufficient water has leaked into the cylinder that piston compression becomes impossible. Massive, and usually terminal, damage will result as you try to start the motor.

Inspection is always a good idea, and we recommend that you try to do so at least every two to four years. It may seem a little pointless since it’s impossible to see into all the passages, but you will still get some clues about the extent of any corrosion. Your manifold may be blocked with the products of corrosion, for example, leading to “hot spots” in the cooling system and low-level overheating. These won’t necessarily show up on your temperature gauge, but can result in a shortened engine life. Inspection is often a difficult job, requiring removal of plumbing components, removal of other components that may block removal of the manifold, removal of the heavy manifold and careful replacement of gaskets.

If your boat suffers more than average corrosion problems, then it may be more likely to suffer from premature manifold failure. Stray current corrosion is evidenced by rapid zinc wastage, corroding lower units and corrosion build-up on through-hull fittings. This is normally the result of poorly installed 12vDC wiring or faulty 12vDC equipment. Electrical problems should be corrected as soon as possible; the damage caused by leaky 12vDC wiring is potentially far worse than a broken manifold.


Replacing a manifold is certainly a job that you can do yourself. It may take longer than you expect, though. Bolts may be rusted in place and other fittings may be in your way. It can turn out to be a day-long job. If this does not appeal to you, boatyards will tackle the job for you.

If you need to replace a manifold you can purchase one from the manufacturer of your engine but you can probably also get one from an after-life manufacturer for a lot less money. And it may be built to the same specs. Ask about this. You will need the make, model and serial number of the engine, the exhaust pipe diameter, and will need to order the necessary gasket kit, installation kit, and end plates if required.

A final word of warning: each year, owners (and mechanics) sink boats by leaving the exhaust ports unplugged after disconnecting the manifolds. Waves sloshing into the ports can sink a boat overnight. Hoses must be plugged and tied securely above the waterline to prevent water from flowing into the boat. With this in mind, consideration for your manifold will pay dividends in reliability and peace of mind, giving you seasons of trouble-free service.

Article courtesy of Boat US

Inspecting Exhaust Manifolds & Risers

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 30′ 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 (e.g., 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. Each year, both owners and mechanics sink boats by leaving exhausts open.

Article courtesy of Boat US

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