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Engine noise diagnosis method
When troubleshooting engine noise, first eliminate noise caused by accessories, such as those generated by air compressors and power take-offs, and do not mistake these noises for engine noise. Remove accessory drive belts to eliminate noise caused by these devices. Noise can also spread to metal parts that are not problematic. Therefore, the use of a stethoscope will help determine where the engine noise is generated.
If you hear changes in the crankshaft speed (engine speed), the noise may be related to the crankshaft, connecting rod, piston, piston pin. If the noise heard changes with the camshaft speed (half engine speed), the noise may be related to the valve train components. Hand-held digital tachometers can help determine if the noise is related to components that operate at crankshaft or camshaft speeds.
Engine noise can sometimes be isolated by removing the plunger of one injector at a time. If the noise volume decreases or disappears, the noise is related to the particular engine cylinder it is in.
There is no fixed rule or test method to determine the source of noise.
Engine-powered components and accessories, such as gear-driven fan clutches, hydraulic pumps, belt-driven chargers, air-conditioning compressors, and boosters, can all make up the noise of the engine. The following information can be used to guide the diagnosis of engine noise:
â— main bearing noise
The knocking noise generated by the loosening of the main bearing can be heard when the engine is under load, and it is loud and boring. If all the main bearings are loose, you will hear loud and short knocks. This knocking sound is regular and varies with the speed. This type of noise is loudest when the engine is dragging on a load or running at a heavy load. The sound of this tapping is duller than the noise produced by the connecting rod. Low oil pressure will also accompany this situation.
If the bearing is not loose enough to make a separate knocking sound, if the oil is too thin or there is no oil on the bearing, the bearing will produce knocking noise.
An irregular noise may indicate wear of crankshaft thrust bearings.
Intermittent sharp knocks indicate that the crankshaft is too large toward the gap. Repeated clutch engagement may cause this noise to change.
â— Connecting rod bearing noise
Excessive link clearance can cause knocking noise at various engine speeds, ie idling and load conditions. When the bearing begins to become loose, the noise may be confused with the slap of the piston or the noise of the loose piston pin. The volume of noise increases with the speed of the engine. Low oil pressure will also accompany this situation.
â— Piston noise
The most difficult part is to point out the difference between piston pin, connecting rod, and piston noise. Double-clicking sounds caused by a loose piston can usually be heard when the engine is idling. When the injector of this cylinder is removed, this striking sound will change significantly. However, on some engines, when the vehicle is driving on the road at a steady speed, the sound of such tapping becomes more pronounced.