ferently under load. You’ve
ruled fuel out as a possible problem, so I suggest you take a look at air
intake and exhaust. Could
you have a blockage in the
exhaust system, perhaps a
kinked hose? How about
your intake filter or screen?
I worked on a boat where
the alternator pulley was
severely misaligned and created so much belt debris that it eventually clogged the intake
screen to the point where there was not enough air to keep the
engine running at normal rpm.
3. Carbon buildup — This is caused by the way we use our
engines. Diesels love high-load, max-rpm running habits, which
we don’t do very often. This promotes carbon buildup in the fuel
injectors and exhaust-mixing elbow where the raw water is injected
into the dry exhaust. Rough idling is one symptom; low rpm under
load can be another. If this is suspected, have the fuel injector
spray pattern checked by a qualified Yanmar shop. You can inspect
the injection elbow on your own with a flashlight and sometimes
a small dental mirror.
engine that’s causing water to leak from the screw cap above the
heat exchanger. The cap allows access to the raw-water screen.
The screen is clear. The discharge out the stern is about half
the flow of the port engine and steam instantly comes out with
exhaust when the engine is started, whereas the port engine discharges only water. What could be causing the flow restriction?
Someone mentioned that mineral deposits can accumulate inside
the system and that there may be a solution that can be added to
dissolve them. — Tim Sergent
San Francisco, CA
I own a 1982 32-foot Bayliner with twin gas Volvo AQ 140,
four-cylinder inboards. I’ve been restoring the boat for the past two
years. There is an issue in the raw-water system on the starboard
Tom Neale: There may be several different things going on.
One possible cause is that there’s a leak in your heat exchanger
(usually in the tube stack), allowing raw water to migrate through
the leak into the freshwater side. When you say “screen,” I assume
you’re referring to the tube stack inside the heat exchanger component. This might cause the water to rise and push out of the
freshwater fill cap. The decreased water flow at the stern exhaust
normally would be caused by something else, such as a bad impeller in your engine’s raw-water pump, or blockage, such as might
be found at the injection nipple where water is injected into the
exhaust. Pull the hose at that point and, with a small flashlight,
check inside the nipple for blockage.
However, the “steam” with the exhaust implies other issues.
It could be caused by the engine overheating, but I assume you
would’ve noticed this from the temp gauge. Or it could be from a
breach in your head gasket or in the casting, allowing combustion
gases to get into the raw-water side. This sounds likely as you say
the “steam” appears instantly. This happened to me once and I
started producing a lot of “steam” (probably combustion gas) at
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