Knock Knock

Maintenance Q & A — July 2002
Maintenance Q & A — July 2002
By Capt. Ken Kreisler

Knock Knock
Detonation and how to avoid it, a plugged oil filter, and more.
 More of this Feature
• Part 1: Detonation, Oil Filter, and more
• Part 2: Charging Voltage, Epoxy Failure, and more

 Related Resources
• Maintenance Q&A Index

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

This article originally appeared in the January 2003 issue of Power & Motoryacht magazine.

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