The existence of the most common abnormal noise of the engine depends on the speed state of the engine.

1, abnormal noise only exists when idling or running at low speed. The causes of noise are: the gap between piston and cylinder wall is too large; The piston pin is assembled too tightly or the connecting rod bearing is assembled too tightly; The gap between tappet and its guide hole is too large; Wear of valve cam profile; Sometimes the starting grip is loose, which will make the pulley ring (obviously when shifting).

2. Keep a certain speed of sound disorder, and make short sounds one after another when slowing down sharply. The causes of noise are: the camshaft timing gear is broken or its fixing nut is loose; The crankshaft is broken; Loose piston pin bushing; Excessive axial clearance of camshaft or loose bushing.

3. Abnormal noise occurs when the engine accelerates rapidly, and it still exists when the engine runs at high speed. The causes of noise are: the connecting rod bearing is loose, the bearing bush melts or rotates because of the inconsistent size; Crankshaft bearing looseness or bearing bush burning; Piston pin is broken; The crankshaft is broken.

Second, the relationship between abnormal sound and load

Many abnormal noises on the engine are obviously related to its load. In the process of diagnosis, the load can be reduced one by one. Single-cylinder or double-cylinder fire extinguishing methods are usually used to reduce the load of one or two cylinders to determine the relationship between abnormal noise and load.

1, a cylinder caught fire, and abnormal sound did not exist or decreased. The reasons for the ringing are: the piston knocks on the cylinder; Connecting rod bearing is loose; Piston ring leakage; The piston pin is broken.

2, a cylinder is on fire, the sound is aggravated, or there is no sound, but there is sound at this time. The reasons for the noise are as follows: the copper sleeve of the piston pin is loose; The taper of piston skirt is too large; The piston pin is disengaged; The fixing bolt of connecting rod bearing cover is loose, and even the alloy of connecting rod bearing bush melts and falls off; The flywheel fixing bolt is too loose.

3. Abnormal sound caused by fire of two adjacent cylinders is reduced or disappeared. The reason for the noise is that the crankshaft bearing is loose.

Third, the relationship between abnormal sound and temperature

1. It makes sound at low temperature, and the sound decreases or even disappears when the temperature rises. The causes of noise are: the gap between piston and cylinder wall is too large; Due to the depth and width of the main bearing oil groove, the piston is misaligned; Low oil pressure and poor lubrication.

2. There is noise after the temperature rises, and the noise decreases or disappears after the temperature decreases. The reasons for the noise are: overheating causes premature combustion; The long and short axes of the piston skirt ellipse are opposite; The ellipticity of the piston is small, and the gap between the piston and the cylinder wall is too small; Piston deformation; The clearance of piston ring is too small.

Fourth, the relationship between abnormal sound and engine working cycle

The abnormal sound fault of engine is often obviously related to the engine working cycle, especially the abnormal sound of crank-connecting rod mechanism and valve train is related to the working cycle. As far as the four-stroke engine is concerned, all sounds caused by crank-connecting rod mechanism will be emitted twice when the engine is working; All noise generated by the valve train is generated by the engine working once.

1. The abnormal sound of crank-connecting rod mechanism is caused by the following reasons: the piston hits the cylinder wall; Knocking sound of piston pin; Cylinder head at the top of piston; Connecting rod bearing is too loose; The piston ring leaks air.

2. Abnormal sound of valve train is caused by the following reasons: excessive valve clearance; The gap between tappet and its guide hole is too large; The cam profile is not damaged; The gap between the valve stem and its conduit is too large; The valve spring is broken; Radial fracture of camshaft timing gear; Loose valve seat ring; The valve is stuck and cannot be closed.

3. If the abnormal sound has nothing to do with the duty cycle, pay attention to its ringing area. The noise caused by the clearance that is usually irrelevant to the working cycle is mostly the failure of engine accessories; If there are parts that have nothing to do with the working cycle that emit continuous metal friction sound, it can be considered that some rotating parts are faulty.

V. Relationship between Abnormal Sound and Engine Parts

When the engine has abnormal sound, it will inevitably produce some vibration. According to the characteristics and location of vibration, the cause of abnormal sound can be diagnosed.