Car and Motorcycle Engine Noise Diagnosis

More often than not, an engine malfunction will reveal itself first as an unusual noise. This can happen well before the problem affects the driveability of the vehicle. Problems such as loose pistons, badly worn rings or ring lands, loose piston pins, worn main bearings and connecting rod bearings, loose vibration damper or flywheel, and worn or loose valve train components all produce telltale sounds. Unless the technician has experience in listening to and interpreting engine noises, it can be very hard to distinguish one from the other.

Using a stethoscope helps to identify the source of an abnormal noise.

When correctly interpreted, engine noise can be a very valuable diagnostic aid. Always do a noise analysis before

moving on to any repair work. This way, there is a much greater likelihood that only the necessary repair procedures will be done.

Using a Stethoscope
Some engine sounds can be easily heard without using a listening device, but others are impossible to hear unless amplified. A stethoscope is very helpful in locating engine noise by amplifying the sound waves. It can also distinguish between normal and abnormal noise. The procedure for using a stethoscope is simple. Use the metal prod to trace the sound until it reaches its maximum intensity. Once the precise location has been discovered, the sound can be better evaluated. A sounding stick, which is nothing more than a long, hollow tube, works on the same principle, though a stethoscope gives much clearer results. The best results, however, are obtained with an electronic listening device. With this tool you can tune into the noise. Doing this allows you to eliminate all other noises that might distract or mislead you.

Common Noises
An important point to keep in mind is that insufficient lubrication is the most common cause of engine noise. For this reason, always check the fluid levels first before moving on to other areas of the vehicle. Some noises are more pronounced on a cold engine because clearances are greater when parts are not expanded by heat. Remember that aluminum and iron expand at different rates as temperatures rise. For example, a knock that disappears as the engine warms up probably is piston slap or knock. An aluminum piston expands more than the iron block, allowing the piston to fit more closely as engine temperature rises.
Also keep in mind that loose accessories, cracked flexplates, loose bolts, bad belts, broken mechanical fuel pump springs, and other noninternal engine problems can be mistaken for more serious internal engine problems. Always attempt to identify the exact source before completing your diagnosis. In most cases, the source of internal engine noises is best identified by tearing down the engine and inspecting all parts.

Abnormal Combustion Noises
Noises caused by detonation and preignition are caused by abnormal engine combustion. Detonation knock or ping is a noise most noticeable during acceleration with the engine under load and running at normal temperature. Detonation occurs when part of the air-fuel mixture begins to ignite on its own. This results in the collision of two flame fronts.

Detonation

One flame front moves from the spark plug tip. The other begins at another point in the combustion chamber. The air-fuel mixture at that point is ignited by heat, not by the spark. The colliding flame fronts cause high-frequency shock waves (heard as a knocking or pinging sound) that could cause physical damage to the pistons, valves, bearings, and spark plugs. Excessive detonation can be very harmful to the engine.
Detonation is caused by excessively advanced ignition timing, engine overheating, excessively lean mixtures, or the use of gasoline with too low of an octane rating. A malfunctioning EGR valve can also cause detonation and even rod knock. Another condition that causes pinging or spark knocking is called preignition. This occurs when combustion begins before the spark plug fires.

Preignition

Any hot spot within the combustion chamber can cause preignition. Common causes of preignition are carbon deposits in the combustion chamber, a faulty cooling system, too hot of a spark plug, poor engine lubrication, and cross firing. Preignition can lead to detonation; however, preignition and detonation are two separate events. Preignition normally does not cause engine damage; detonation does. Sometimes abnormal combustion will result in another type of noise. As an example, rumble is a term used to describe the vibration of the crankshaft and connecting rods caused by multisurface ignition. This is a form of preignition in which several flame fronts occur simultaneously from overheated deposit particles.