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1. Valve knock
There is a crisp metal knocking sound in the cylinder head cover. When the diesel engine is shifting at medium and low speeds, the knocking sound is more obvious and crisp. There is a regular "beep, bang" sound, mainly due to excessive valve clearance. When the diesel engine temperature rises or is tested by the "broken cylinder" method, the sound does not change. When the diesel engine is running at high speed, the knocking sound is not obvious because other mechanical noises are large. The main causes of excessive valve clearance are: the valve adjustment screw is loose, so that the screw is withdrawn or the moving parts of the valve train (such as cam, tappet, rocker arm, etc.) are worn. When the valve clearance is too large, the diesel engine may be under-inflated due to insufficient intake air or insufficient exhaust gas, causing power reduction, fuel consumption increase, etc., and the valve clearance should be adjusted in time.
2, the piston pin
The main reason for the piston pin is that the piston pin, the connecting rod bushing or the piston pin seat hole wears, causing the matching clearance to be too large, and the piston pin vibrates during the movement to make a sound. The characteristics of the piston pin are: when switching from idle speed to medium speed, the sound is louder; the period (frequency) of the sound changes with the rise and fall of the engine speed. When the injection time is advanced, the sound is more obvious and clear. After the engine temperature rises, the sound is generally not weakened, and sometimes it is significantly enhanced. Tested by the broken cylinder method, the sound is weakened or disappeared.
3, the piston knocks the cylinder
The common and main reason for the piston knocking cylinder is that the piston and the cylinder wall are worn out, and the clearance exceeds a certain limit. The piston oscillates during the up and down movement, and the cylinder wall is struck to make a sound. Sometimes, the matching clearance does not exceed the limit, and if the connecting rod is bent and deformed, it will sound. When the temperature of the diesel engine is low, the sound of the piston knocking cylinder is obvious, especially when it is idle. As the temperature rises, the sound decreases or disappears. This is the characteristic of the piston knocking cylinder. Since the knocking sound and the piston pin are similar, in order to facilitate the difference, a small amount of oil can be injected into the top of the piston from the injector seat hole, and after several revolutions of the crankshaft, the diesel engine is started again. If the sound disappears at the moment after starting, it is the sound of knocking. It should be noted that when the oil supply is advanced (within the specified range), it also causes a slight knocking sound, which is normal.
4, the connecting rod bearing ringing
The main reason for the linkage of the connecting rod bearing is the wear of the journal and the bearing, the bearing alloy layer burnt out, the bearing cap (the tile cover) is loose, the bearing rotates (the bearing bush goes outside the circle), and the moments of the work are mutually impacted and sounded. The characteristics of the connecting rod bearing ring are: the sound is loud at medium speed, and the sound is not obvious due to the increase of other mechanical noise at high speed. When the temperature of the diesel engine changes, the sound does not change. In the medium speed range, when adding or decelerating, the sound will increase as the speed increases. In the broken cylinder test, the sound becomes smaller or disappears.
5, crankshaft bearing ringing
The reason for the crankshaft bearing ringing is the same as that of the connecting rod bearing. Its characteristics are: the sound is louder than the knocking cylinder, the piston pin and the connecting rod bearing. When the fuel is suddenly added or suddenly decelerated, the sound is clearly clear; if the bearing alloy layer burns out or falls off, the diesel engine will shake. When the operation is heavy, the sound is clear and clear. The sound does not change when the engine temperature changes. When the bearing ringing occurs, the single-cylinder "broken cylinder" test has no obvious change in the sound; the adjacent two cylinders "breaking the cylinder" test, the sound will become smaller or disappear. In addition, you can use a metal rod or a wooden handle screwdriver to touch the bearings of the external parts of the diesel engine.
Note: If the sound is caused by the excessive gap between the bearing and the bearing neck, the oil pressure will drop significantly.
6, camshaft bearing ringing
The common and main reason for the camshaft bearing ringing is that the camshaft and the bearing (sleeve bushing) are worn out, and the matching clearance is too large; the bearing alloy layer burns out, falls off or the bearing outer diameter and the seat hole bearing "take the outer circle". The camshaft bearing ring is an intermittent, rhythmic, blunt "beep, hum" medium speed, the sound is obvious, and can be heard at idle speed. When the engine temperature changes or the cylinder break test, the sound does not change.

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