|
What
causes detonation, and how can it be avoided? S.W., via e-mail
Commonly
known as “spark knock” or “fuel knock,” it’s
the violent explosion, as opposed to the even burning, of fuel in the
combustion chamber. As illustrated above, both pressure and temperature
in the combustion chamber rise dramatically, creating severe shock waves
that can cause major damage.
One
common cause is using fuel with an insufficient octane rating, but detonation
can occur even with high-octane fuel, due to a variety of factors. These
include ignition timing that is advanced too far, too lean a mixture (i.e.,
too much air per unit of fuel), and overloading, perhaps due to using
a prop with too much pitch. Older engines may detonate due to excessive
carbon deposits that heat up and ignite the fuel-air charge before the
spark plug.
During
a recent oil change on my diesel, I noticed an unusual amount of debris
in the oil. What could have caused this? P.T., via e-mail
Assuming
that you change your oil at proper intervals and that your engine hasn’t
been subject to any unusual operating conditions (i.e., overloading or
overfueling), you may have a plugged oil filter element. All diesels naturally
generate a variety of byproducts during combustion, ranging from metals
to carbon. Lube oil is designed to hold these contaminants in suspension
until the oil reaches the oil filter, which removes them. If the filter
element becomes saturated for any reason—normally due to an unusually
large amount of contaminants being generated—the element becomes
clogged and oil will not pass through it. At this point, a bypass valve
opens to route the oil around the filter to maintain the necessary supply
to critical engine parts. Unfortunately, this oil continues to acquire
contaminants until it is drained. Paying strict attention to maintenance
schedules for your engine is the best way to avoid this.
What
is the proper maintenance for a raw-water pump? M.B., via e-mail
Since
the most likely point of failure is the impeller, you should visually
inspect the raw-water pump impeller annually, along with hoses, connections,
hose clamps, and through-hulls. Most mechanics agree that disassembling
the pump and its parts and removing the impeller, although offering a
better opportunity to see problems, often creates more problems than it
solves. Unless you believe your impeller has suffered damage from ingesting
sand or grit or run dry for any length of time, leave it alone.
A better
diagnostic strategy is to note the amount of water that normally exits
the exhaust system at idle. If you feel this has diminished, take a closer
look at the impeller.
Next page
> Charging Voltage, Epoxy Failure, and more
> Page 1, 2
|