More of this Feature • Part 1: Lift-type Muffler, Engine Noise • Part 2: Galvanic Corrosion, Misfiring Outboard, and more
Related Resources • Maintenance Q&A Index

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