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What’s That Engine Noise?

Wednesday, May 1, 2013

Have you ever had an engine with a noise that you just couldn’t diagnose? Different engine designs have different characteristics that can cause an engine noise that is not necessarily an indication of a pending failure. The following is a quick guide that will cover the majority of engine noises and their causes.

Knock occurring at half engine RPM or camshaft speed:
Hydraulic valve lifters are a common source of engine noise. New lifters may take a few minutes to pump up with oil and will be noisy until they are pressurized. A bad lifter can stick or simply be defective from the manufacturer. A faulty lifter can be isolated by listening to each rocker pivot point with a stethoscope while the engine is at idle. Also, a similar noise can occur if you have a damaged valve spring, pushrod, rocker arm or even excess valve lash. Even mechanical fuel pumps can make a similar noise at valve train speed.

Deep metallic knock:
This noise is caused by excessive main bearing clearance. The noise is most audible when the engine is under load, accelerating or when it is just started. Low oil pressure always accompanies excess main bearing clearance because naturally as clearance increases oil pressure decreases. This noise must be addressed quickly because failure to address the issue early can result in a spun main bearing. If this occurs the engine block will need to be align bored and this is an additional charge at any engine remanufacturer.

Sharp metallic rap when releasing or engaging manual transmission clutches:
If crankshaft endplay is beyond the factory specifications this is the noise you will hear. This can be diagnosed by placing a dial indicator at either end of the crankshaft. Carefully pry the crankshaft from one side to another recording the total limits of travel. It’s as simple as comparing that number to the factory specification and if it exceeds then there is a problem with the thrust of the crankshaft.

Metallic rap when the engine is running under a light load:
This is caused by excessive connecting rod clearance. The noise will become louder and will increase in frequency as the engine speed increases. You can isolate the defective connecting rod bearing by removing spark from one cylinder at a time. When spark is removed from the defective cylinder the metallic rapping will noticeably decrease. The combustion is what thrusts the piston and rod downward causing this engine noise.

Dull metallic rattle heard at idle and under light engine load:
Piston slap is the noise associated with excess piston-to-cylinder wall clearance. On some engines the noise can disappear when the piston expands during warm-up. In this case the noise will not affect the engine’s reliability. However, if the slap remains after warm-up, piston or ring failures are possible. In this case, grounding a spark plug will not affect the noise.

Light metallic rap at idle and at low speeds:
This type of noise is caused by excessive piston pin clearance. Similar to excessive rod clearance noisy piston pins are isolated by grounding spark plugs. However, the affect is different than too much rod clearance. Noise from a loose piston pin will actually increase in frequency with the spark grounded. This noise typically occurs after piston rings are replaced due to the additional drag on the loose pin.

While these are some of the most common noise sources, remember that noises can come from areas other than internal engine parts. Drive belts, alternators, compressors, air pumps, and fuel pumps all make noise. This is why it is good practice to first disconnect the accessories and run the engine to determine if the noise disappears. To prevent overheating be sure to only run the engine for a short period of time.

Material covered in this article came from various engine manuals and handbooks. All Industrial Engine Service is not responsible for any damage or failure caused by the techniques mentioned in this article. If you have any questions regarding this or any other forklift related issue please feel free to give us a call 877/303-LIFT (5438).

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