Correct diagnosis of engine noise

The abnormal sound of the engine is mainly caused by the wear and looseness of the internal parts, improper adjustment, or improper use. It means that the engine has large or small faults. It should be diagnosed in time and effective preventive maintenance measures should be taken.

First, the initial diagnosis, determine the impact of abnormal noise on the engine

In many complex engine sounds, it is first necessary to determine which are normal sounds and which are abnormal sounds. Which of the abnormal noises have little effect on the working conditions of the engine is allowed to exist temporarily; which ones are not allowed to continue to exist, they must be eliminated immediately, otherwise it is more likely to cause greater harm.

1. If the sound is slightly simple when the engine is running at a low speed, the roar sounds even and smooth during high-speed operation, and it appears as a smooth transition during acceleration and deceleration, which is a normal sound.

2. If the sound is only present at idle speed, it will disappear after the speed is increased. There will be no noticeable change in the sound during the entire use. This is an unusually noisy sound that allows for temporary existence and should be repaired at the appropriate time.

3. If the sound is suddenly accelerated or suddenly decelerates, and when the engine does not disappear during the medium and high-speed operation, and the organic parts vibrate at the same time, then this is an abnormal sound that is not allowed to continue to exist and should be checked immediately. Explain why and exclude it.

4. If the sound suddenly appears during the operation, and if it is more violent, it should not be continued or diagnosed by trial-and-error. Instead, it should be immediately shut down for inspection. Generally, the oil pan is disassembled first, and then the cylinder head and the valve chamber cover are removed.

Second, careful analysis, and then identify the site, cause and degree of abnormal sound

In terms of the appearance of abnormal noise and the duration of continuous existence, abnormal noise generally exists in several periods such as idle or low-speed operation, during high-speed operation, and during the entire journey.

1, for abnormal sound during idle or low speed operation

(1) First, check whether the abnormal sound is related to the work cycle, and further determine which mechanism the fault is in. If there is an association, it indicates that the failure occurred in the crank linkage mechanism or the valve distribution mechanism. According to the frequency of abnormal sounds, a preliminary determination can be made. All the sounds caused by the crank connecting rod mechanism are emitted by one working cycle twice, and the sound caused by the gas distribution mechanism is emitted once by one working cycle. Therefore, the crankshaft bearing Frequencies of abnormal sounds at places etc. are low, and frequencies of abnormal sounds at valves, etc. are high.

(2) Gradually increase the engine speed, hear whether there is any abnormal sound, and judge the degree of wear of the moving parts based on the change of the abnormal sound with the speed.

(3) In the process of diagnosis, attention should also be paid to observing the effect of changes in engine temperature on abnormal noise.

(4) For a multi-cylinder engine, check whether the abnormal sound and the cylinder position are related by using a single-cylinder cut-off method. If there is a noticeable change in abnormal noise after a cylinder breaks the oil and breaks the fire, it means that the fault is in this cylinder; if there is no obvious change in the abnormal sound after a cylinder breaks the oil and breaks the fire, it means that the abnormal sound is not related to the cylinder position.

Through the diagnosis of the above-mentioned several processes, the relationship between abnormal noise and engine load, work cycle, rotation speed and temperature can be basically ascertained. If the abnormal sound matches certain abnormal sound characteristics, a final diagnosis can be made.

2, for the abnormal sound during high-speed operation

(1) The engine speed is gradually increased from low speed to high speed operation. In this process, pay attention to the timing of abnormal sound.

(2) When abnormal noise occurs, it is stable at this rotational speed and the abnormal noise is carefully observed. Sometimes abnormal sound may occur only in the case of sudden acceleration or rapid deceleration, then the speed of the engine should be rapidly changed to find out the timing of abnormal sound.

(3) In the process of diagnosis, attention should also be paid to the accompanied phenomenon change of oil pressure, oil filling port, exhaust pipe, etc., to assist in diagnosing the malfunction, and to reach a final diagnosis.

3, for the abnormal sound during driving

The engine abnormal noise in driving can generally be reflected after the engine is stopped in the same speed, which leads to the diagnosis of abnormal noise. But sometimes there are exceptions, unusual noise in the car, after stopping, the engine running at the same speed, but no longer appear this abnormal sound, then you should change the speed of rapid changes, generally can make abnormal sound. Then find the reason and draw a final diagnosis.

Sometimes the abnormal noise in driving may not be abnormal sound of the engine, but may be the abnormal sound of other parts. You should press the clutch pedal or disengage the transmission gear, and then do the rapid acceleration test. If it disappears, it means that the abnormal sound is not in the engine but in the chassis or body part.

The diagnosis of engine abnormal noise is an experienced maintenance job. Because the abnormal sound of the engine is various and intricate, the abnormal noise reflected by different faults may be similar, or the same fault may be due to different degrees It also reflects different sound characteristics. Normal sounds and abnormal sounds are mixed and difficult to distinguish. Diagnostic personnel only have to conduct thorough research, repeated analysis and identification, in order to master its regularity, and then actively diagnose abnormal sound.

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