A Audi C5A6 car, equipped with 1.8T engine and automatic transmission, has a mileage of 100 thousand km. The car owner reflected the engine in the 2 and 3 gear acceleration when there was a "no" noise, the sound had lasted for about half a month, and sometimes it was not, until these two days the sound became obvious. It was found that the sound of the engine was, as stated by the car owners, when the 2 and 3 gears were accelerated to hear the engine go out, and the sound was really like the explosion of the engine, but the engine's power was not affected, and the sound of the engine was normal. In combination with fault analysis, the engine can produce a "click" sound when the engine accelerates in the 2 and 3 gears, mainly for the following reasons. (1) the fault of variable valve timing mechanism. Because the engine has entered a medium load condition when the 2 and 3 gear accelerates, according to the characteristics of this engine, the timing of the valve should have entered the adjustment position, and the adjustment mode is realized by changing the position of the chain tensioning wheel. According to the principle, when the camshaft chain is adjusted, if the strength of the chain tensioner is insufficient, the chain will make a sound in the relaxation state during the adjustment process. (2) the sound produced by a detonation of the engine. There are the following possibilities: (1) the low octane number of fuel can cause knock. The engine of this vehicle should use more than 93 lead-free gasoline. It is best to use more than 95 lead-free gasoline, otherwise it will cause knock. The ignition time is prematurely produced by detonation. Because the engine adopts the independent ignition system controlled by the computer, the error of the engine's computer adjustment leads to the possibility of premature detonation of the ignition. 3. The coke on the top of the piston and the combustion chamber cause the actual compression ratio to be too large and cause knock. (4) abnormal adjustment of turbocharging system. The vehicle is equipped with a turbocharging system, and the structure and working principle of the engine supercharging system are shown in Figure 2. When the engine does not need to be pressurized, the vacuum in the intake manifold passes through the vacuum tube through the air recirculating solenoid valve to suck the diaphragm of the air recirculating valve so that the turbocharger exit is interconnected with the inlet of the supercharger, and the supercharger does not function. When the engine is overspeed cut off, in order to ensure that the air recirculation valve is reliably open, the air recirculating solenoid valve switches off the air recirculation valve, connects with the intake manifold, and connects with the vacuum tank so that the air recirculation valve can not be opened because of the low vacuum in the intake manifold. The pressure control valve and the pressurized pressure control solenoid valve are used to control the size of the engine pressure. When the turbocharging system pressurized the engine when the turbocharging system should not be pressurized, the pressure in the cylinder would be too high, and the engine control unit did not adjust the ignition system to the ignition system. It will lead to "detonation" in the process of acceleration. Through the analysis of the reasons for the possible abnormal sound, the various possibilities of the failure have been basically clear, and the author decides to exclude the fault in the order of simplicity to propagation. The plug of the valve timing control solenoid valve is pulled out so that the vehicle can not adjust the valve timing. Then the test was carried out, and the failure was still found, which ruled out the possibility of the noise caused by the shortage of the chain tension when the gas phase was changed. It seems that the cause of the abnormal noise should be caused by the detonation. After asking the owner, the car has been using No. 93 unleaded gasoline in a fixed gas station, and not only his own car, all the cars in this unit are refueling at that gas station, most of them are the same type of car, and there is no problem after refueling. Therefore, for the possibility of detonation caused by low fuel octane number, I decided to doubt it again. Use the special fault diagnosis instrument V.A.G1552 to retrieve the engine trouble code, and there is no fault code display. Reading the data flow and driving the car owner to be able to send out the abnormal sound, the emphasis is to read the pressure pressure and key to the 01-08-115 on the V.A.G1552. The third zone is the theoretical pressurization pressure, and the fourth is the actual pressure pressure measured by the pressure sensor at the medium cooler. When the car owner drives the vehicle to speed up slowly in the 3 gear to 40km/h, the theoretical pressurization pressure of the third area is 101kPa from the third zone, and the actual pressure of the fourth area has reached 127kPa. At this time, the sound of the engine has been heard. The cause of the problem is that the actual pressurization pressure is too large. The deflagration of the engine and the over pressurization pressure may be caused by the failure of the air recirculation valve. First, we should check the vacuum source of the air recirculation valve. Remove the hood from the engine and find that the vacuum tube that connects the intake manifold and the air recirculating solenoid valve has become crisp because the heat from the engine has been crisp. After the replacement of this vacuum tube to test the car again, in the 3 block to 40km/h and slow acceleration, the actual pressure of the V.A.G1552 fourth area pressure has dropped to 110kPa, the sound of deflagration also disappeared, but again when the acceleration will appear some very light "La, La" sound, at this time the actual pressure pressure is also normal, no more than 110kPa . The reason for this problem is that because of the high pressure in the cylinder in the last half month, the engine has been in a state of overheating, resulting in carbon accumulation in the combustion chamber and the top of the piston. The actual compression ratio of the cylinder is higher than that of the theoretical compression ratio. And the possibility of detonation. In order to solve this problem, the coke cleaning equipment was used to clean the coke in the inlet, valve and cylinder. After first cleaning, second times were cleaned after a period of time. After that, the fault was thoroughly solved.