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How
does a lift-type muffler work, and how is it maintained? W.A.,
via e-mail
Usually
found on low-horsepower powerplants used for main propulsion and genset
engines, the lift-type muffler is a vertical canister that fills with
raw water and gas from the engine’s exhaust system until it reaches
a sufficient level to overflow and exit via the boat’s exhaust system.
The water cools the gases and reduces sound levels.
Although most lift-type
mufflers are not serviceable, you should regularly inspect all fittings
leading to and from it, ensuring hoses are free of abrasions and hose
clamps are tight.
Can
engine noise indicate the source of an internal problem on an inboard
gasoline engine? T.R., via e-mail
Yes.
In fact, a strange noise is often the first evidence of something wrong
inside an engine. Many sounds are obvious, such as spark knocking and
rattles associated with loose parts, but there are more subtle sounds
that can also indicate potential problems.
Professional mechanics
often use a stethoscope to locate and identify internal noises not usually
discernable to the human ear. If you don’t have one of these, you
can use a length of dowel or "sounding stick" or even a piece
of tubing. Simply place one end on the engine in the suspect area and
the other end on your ear. While you may find it difficult to identify
and interpret what you hear, the following can be used as a guide.
Clicking or tapping
noises, for example, usually come from the valve train and are indicative
of excessive valve clearance. A sticking valve may also make the same
sounds as a valve with excessive clearance. In addition, excessive wear
in valve train components can cause similar engine noises.
A heavy, dull knocking
is usually caused by a worn main bearing. It will be loudest when the
engine is working hard, such as during acceleration or under load. If
you have an older gasoline engine without electronic fuel injection, you
may be able to pinpoint trouble in a single bearing by disconnecting the
spark plugs one at a time. When you reach the plug nearest the worn bearing,
the knock will reduce or disappear.
Worn connecting rods
may also produce knock, but in this case the noise will be more metallic.
As with the main bearing, it will be worse during load and may increase
when you reach cruising speed. Disconnecting spark plugs will also help
isolate this problem.
A double knock or clicking
usually indicates a worn piston pin. Disconnecting individual spark plugs
will isolate this to the problem piston, but the noise will increase when
you reach the right one.
A loose flywheel and
excessive crankshaft end-play also produce knocking sounds. While similar
to main bearing noises, they are usually irregular and do not change when
you disconnect the spark plugs. This type of knock is generally heard
at idle or during rapid deceleration.
Piston pin noise and
piston slap–indicative of excessive piston clearance–are often
confused. The double knock will distinguish piston pin noise, while piston
slap is always louder when the engine is cold.
After listening to your
engine, should you suspect something is wrong, call in a qualified marine
mechanic and give him your impressions. This not only helps to isolate
the problem, but with the clock running will also help keep your bill
lower.
Next page >
Part 2: Galvanic Corrosion, Misfiring Outboard, and more
> Page 1, 2
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