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All
marine inboard engines have exhaust risers. The exhaust riser has
two functions: (1) to keep water from backing up into the
engine through the exhaust piping and, (2) it serves as the device
that introduces cooling water into the exhaust exit piping system.
If the riser fails to accomplish either of those jobs successfully,
the end result is the arrival of Uncle Big Bill on the scene.
As someone who has evaluated hundreds of engine
damage insurance claims, I can tell you that exhaust riser failures
-- either in the design of the thing, or by corrosion failures --
is the single largest cause of engine damage and premature engine
failure.
Riser problems come in two flavors: Corrosion failures
and design failures. If you wish to keep Uncle Big Bill at bay,
you need to understand the following points.
Corrosion Failures
All common gasoline engines use cast iron risers, as do most small
auxiliary sail boat engines. Cast iron, of course, corrodes, and
is a lousy material to make risers with. Problem is, there isn't
anything better. At least not that you're willing to pay for. If
you have extra $$$, have them custom machined from titanium and
you will solve the problem. That would set you back about $5,000
apiece, so in the meantime we're stuck with cast iron.
Actually, for the low cost, cast iron works pretty
well. But, like the tires on your car, they do not last indefinitely.
Depending on how well they are designed -- for design has an awful
lot to do with it -- some last longer than others. Mercruiser,
for example, has a wonderful reputation for risers that sometimes
don't last a year. On average, in saltwater use, good risers last
about 4-5 years. At 4 years, you should start worrying about them.
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| Mercruiser 4.2 V6 riser.
Arrow shows where corrosion hole occurred. It doesn't take
much corrosion to block these small water passages. Note that
the passages top and bottom only have a tiny hole where it
met the larger passages on the manifold. Hard to figure this
one??? |
Merc 350 riser. Notice
the difference, especially the width of the gasket surfaces
between the openings. This one had been cleaned of scale and
put back in service. That didn't stop it from leaking and
ruining the engine, which it did. |
Cast iron works well because it tolerates extremes
of temperature, expansion and contraction better than other options
like stainless steel. Since cooling water is injected into the exhaust
heated part, there is usually a zone of very high temperature difference
-- a transition zone from superheated to quickly cooled. If you
were to heat up a cast iron skillet red hot and then pour cold water
in it, you know that that would probably crack the pan apart. Same
thing with risers. Ergo, how they are designed has a lot to do with
it. The heat transition zones result in more rapid rates of corrosion.
Cast iron corrodes in a unique way, depending on
type, it corrodes with a sort of sheeting action, or a blistering
action, leaving layers of heavy scale on the corroded surfaces.
It is not unusual for that scale to become 1/4" thick. When
the scale gets that thick, it ends up blocking the water flow. The
other problem with this is that this scale stays wet, retains water,
a situation which advances the corrosion process. Unlike aluminum
or Corten steel, the corrosion does not serve to protect the metal,
quite the opposite. Therefore, as cast iron corrodes, the rate of
corrosion becomes progressive. Meaning that the rate of
corrosion becomes faster. Get the picture here? Right.
By the time you see corrosion on the exterior, it's is time to start
worrying.
It is the thickness of the "jacket,"
the wall between the exhaust chamber and the cooling water, that
determines how long it will last. The thinner it is, the sooner
it will fail, but you have no way of know what the thickness is.
So you're sort of stuck waiting for the apocalypse.
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| This Crusader 454 riser
system is 12 years old. That's because the riser is closed
system cooled. Naturally, it costs more money. The water induction
elbow is behind the coolant bottle and is a lesser expensive
throwaway. Notice the oil filter adapter where you can easily
reach it. |
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Crusader riser, salt water cooled. This
design uses a steel insert ring with the water passage all around
it. It too, is narrow and over time became completely blocked
with scale. |
As with engine blocks, risers have water passages
inside that also corrode. Some riser designs, like those of Mercruiser,
have passages that are so narrow that the rust scale can quickly
block those passages. This causes two problems. (1) it reduces cooling
water flow through the whole engine and causes engine overheating,
or (2) it can cause the exhaust system to starve for cooling water,
overheat, and possibly setting the boat on fire. Neither make for
happy days.
Even worse though, if corrosion makes a hole between
the water jacket and the exhaust chamber, then the cooling water
can go down into the exhaust manifold, flood it, into the valves
and then the cylinders. This is a very unfortunate state of affairs
because it can wreck the engine. You don't want this to happen because
it probably is not covered by insurance, meaning you pay. Of course,
you cannot see that hole because it's on the inside. One day you
go to start your engine and nothing happens. The starter clicks
and clunks. Water has seized the engine up. On that day, there will
be much wailing and gnashing of teeth.
The moral of the story here is that you need to
do one of two things. Pay Little Bill to stave off Big Bill. Or
wait and pay Big Bill.
(1) If you have corrosion
appearing anywhere on the riser exterior, it is time to pull them
and have them inspected internally. Replace them if Mr. Goodwrench
recommends that. Trying to salvage wasted risers is not worth the
risk.
(2) At the end of 4 years,
automatically pull the riser(s) and have them inspected, even if
there is no external corrosion. Just because you don't see it on
the outside, doesn't mean that the riser may not be about to fail.
No, it does not matter how many hours there are
on the hour meter. The hour meter measures itself, not the rate
of corrosion and is totally irrelevant to anything. Throw them overboard;
hit them with a hammer; rig them to a bomb, send it to your mother-in-law;
change your oil by them. Whatever. But do not judge the condition
of your engines by them. They don't mean anything short of a meaningless
number.
Aluminum Risers Aluminum
is cheap and easy to cast, and therefore cheap in price. Aluminum
is the wrong material to use for risers, which is all there is to
say on the subject, except that some Yanmar engines have them. Good
luck is needed here.
Leaking Gaskets The risers
bolt to the exhaust manifold, and because the two parts expand and
contract at different rates, it's hard to maintain a constant gasket
seal. If water is leaking to the outside, it is also leaking
to the inside. Count on it. Address the situation IMMEDIATELY.
This may not wreck your engine, but salt water under the riser gasket
will corrode the gasket surface on the exhaust manifold. A gasket
cannot seal to an eroded surface. Once this happens, no gasket will
ever seal again and the manifold has to be replaced. If you thought
risers were expensive, wait till you see the cost of manifolds.
Sail Boats Exhaust systems
are more bizarre and haphazard on sail boats than any other kind.
That's because sail boat builders don't like engines, so they often
don't pay much attention to their installation. it's absolutely
critical that the riser system does not allow water going up the
exhaust -- or that which the cooling system puts there -- does not
get back into the engine. Most systems are okay in terms of height,
it's usually other aspects that fail to meet the test of time.
The best thing for a sail boat is to use a dry,
heavily insulated exhaust riser. That means no water cooling until
injecting water on the down side. The problem you see in the
photo below is that the water is injected on the UP side. This will
eventually make life unpleasant for you. Often combined with water
lift mufflers, this can be a problem if the muffler too can fill
up with water. Personally, I do not like water lift mufflers because
they cause too much back pressure, among other things.There often
isn't enough space to install them correctly. If a system
is designed right, it doesn't need a muffler as the water in the
hose will accomplish all necessary muffling.
The system pictured below is rather typical of
systems installed by what we might kindly refer to as non experts.
If you don't know anything about exhaust systems, it probably looks
fine. If you do know, then it is an obvious disaster waiting to
happen. I find so many half-baked systems like this in sailboats
that it's beyond my ability to summarize in a short essay how to
take care of them. If you are in doubt, have an expert look at it.
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Here's a picture that would make any
surveyor's blood run cold. This cast aluminum riser is guaranteed
to wreck this engine when it fails, as it inevitably must.
In addition, combined with engine vibration, that long lever
arm (pipe between aluminum riser and manifold) is bound to
break the mount or pipe. The insulation on the down side indicates
that the exhaust hose is getting too hot, so they wrapped
it with insulation!!! |
Diesel Engines Most diesel
engines no longer have cast iron risers but stainless steel. Didn't
I just say stainless was not so good? Yes, but changes in design
make it more practical for diesels because people pay more for diesels.
Diesel risers are VERY expensive. We are talking Bill's Granddaddy
here. Sometimes great, great granddaddy.
With turbocharged engines, the exhaust system is
different. And so are the risers. One part of the riser system is
between the exhaust manifold and the turbocharger. This is normally
fresh water cooled and does not figure into our discussion here.
There is another component we also call a riser, and that is the
part that attaches to the exhaust side, or rear of the turbo. On
all Caterpillars since at least 1990, that's going to be a welded
stainless elbow. On Detroit Diesels it may be separate elbows on
each side, or it may be a single, very large "Y" combination
riser/crossover. It doesn't much matter except in price because
the function is the same.
Both the iron and stainless versions are shown
below, along with the obvious reasons why you want the one and not
the other.
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Iron riser for the non turbocharged
Cat 3208. Here the very small water passage holes became completely
blocked within just a few years. Riser failure here wrecked
the engine.. |
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The latest diesel riser designs utilize
this non-cooled section (black arrow) between the water induction
section (white arrow) This precludes the possibility of riser
failure resulting in water going into the turbo. Detroit Diesel
8V92 |
One difference between gas and diesel is that diesel
manufacturers do not provide a riser with the engine. Therefore,
whatever you get is an after market product, about the quality of
which neither you nor I know anything. The good news is that
when diesel risers behind a turbocharger fail, the leak isn't likely
to go into the engine. The fact is that so many engine manufacturers,
boat builders and riser makers got their pants sued off in the 1970's
and 1980's for lousy designs that wrecked expensive engines, that
they decided to resolve the problem once and for all by designing
systems where this wouldn't happen. And so it doesn't, proving once
again that it's cheaper to do it right than wrong.
Diesel exhaust pipes have to be huge because these
engines won't tolerate any back pressure, which will burn up exhaust
valves. The riser, which we can also call a water induction
elbow, does just that; it injects salt water from the cooling
system into the exhaust piping, which is hose or fiberglass, so
that the exhaust doesn't burn it up. And because the pipe is very
large, a special arrangement has to be made to ensure that the top
part of the hose or pipe gets enough cooling water. Otherwise, the
fiberglass, rubber or silicone parts will burn up. That's not good,
so they put a lot of holes all around the induction section like
in the photo below. This causes a spraying action that gets all
areas properly cooled.
But they are small holes, aren't they? And you
know what that means. Small holes normally become big holes, except
when they're not supposed to, like in our riser here. Remember the
case of the "disappearing water pump impeller blades"?
Yes! Now you know where all those broken pieces of water pump impeller
blades end up. They go completely through the system and stop right
here at our risers to make our lives somewhat less than totally
pleasant, and our bank accounts a little emptier.
Yep, lodged right against our little holes there,
very nicely blocking off the water flow right where you need it
most. Naturally, this creates the same problem as with a gas engine,
reducing cooling water flow and increasing tendency to over heat.
So what we do about this then, is to take out our gun and shoot
the damn thing. Angry as hell and not going to take it anymore.
Take that! You miserable chunk of stainless!
An infrared thermometer gun, that is. This allows
us to "scan" the system and find hotspots that will tell
us where the blockage is. Actually, you can even feel around the
exhaust hose (especially at idle) and if there are spots that burn
your hands, you've located the problem because the hose should not
be that hot. Consequently, the moral of the story here is that even
diesel systems need to be checked out occasionally, just like we
know we should do preventative maintenance on our car radiator but
never do, waiting for it to crap out at the worst possible moment.
But car radiators cost hundreds, while we're talking thousands here,
folks. A little Priority Management 101a.02b7, section DXDCIIV,
Title III is in order here. Under the heading. A stitch in time
saves nine.
If the system is not designed right, then it is
possible for water to at least get into the turbo. This is one of
the most common causes of turbo charger failures. You replace a
$2000 turbo, not knowing why it failed, only to have it happen again
because the riser is the culprit. This can happen when the elbow
is at a horizontal angle, so that the water can run forward instead
of only aft. Bearing in mind that a little change in trim of the
boat can help the leaking water along here in positively the wrong
direction. Notice that the stainless riser in the photo above is
angled downward somewhat. Even if it did leak, the water would run
away from the turbo, so this set up is a good one. Very good.
Not all diesel mechanics are
marine mechanics. Many, especially those employed by engine
dealerships, are truck and heavy equipment mechanics with
little or no marine experience. I have found that many do
not understand the effects of saltwater on these engines,
and have made costly diagnostic mistakes regarding exhaust
systems. It is best to employ a marine specialist if you
can find one, but they are not universally available as
they are in major boating centers. |
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Large diesel induction elbow. This
one is 5 feet long. In this photo, the top of the pipe is
at 2 o'clock, the bottom at 8 o'clock where all the rust
stains are. There should not be any rust stains on the inside,
which means that there are leaks. These leaks all
came from weld joints, all of which were repaired and this
riser was put back in service. |
Leaks Stainless exhaust
system parts are welded, and welds are weak parts as far as corrosion
goes. And many of them are only welded from the outside because
it's hard to weld on the inside of a pipe. Ergo, leaks usually
occur at weld joints. One very good thing about stainless is that
it does not generally corrode, but rather pits in unexplainable
random places. Meaning that there can be a little hole smack in
the middle of a bunch of nice clean steel for no apparent reason.
You don't need to know why this happens, only that it does. The
really nice part is that Mr. Goodwrench can take that part off,
back to his shop, and just weld that little old hole up. Presto!
And put it back on. That part may last indefinitely. You like
it and I like it.
Neither of us like it when the experts make big
mistakes. Like attaching a carbon rubber hose to a stainless riser.
Carbon rubber and stainless are like cats and dogs; they do not
get along, and the rubber always wins. That rubber will eat the
stainless right up, and it will leak like crazy.
At the point where the hose attaches to the riser,
another thing happens, and it's not good either. it's called crevice
corrosion. In the simplest of all possible terms, water gets between
the hose and the metal and corrodes it because of a lack of oxygen.
Don't matter how or why, just know that it does. So, if you see
leaks occurring at the point where the hose attaches to the elbow,
it is best to get those leaks stopped right away. Because, if
you don't, you can be sure that like Paul Revere riding through
the night, Big Bill is a comin'. If you wait until the little
leak becomes a waterfall, then you'll be facing the replacement
of an $1,800 elbow (plus labor) times however many are going bad.
Plus whatever that leaking saltwater leaked onto and damaged.
Like that $800 engine starting motor. Save your britches and take
those stitches.
Epilogue Leaks on engines
are nothing to fool around with. They are trying to tell you that
something is wrong, and you ignore them at your own peril. This
is part and parcel of why boat ownership is so dang expensive:
they float in saltwater which is horribly corrosive. While we
can marvel at how fish thrive in this stuff, and relish eating
them, we pay a high price to travel where they do. But it's a
whole lot cheaper to nip these problems in the bud, rather than
letting them grow shark's teeth.
Posted October 23,
1998
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