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A mileage car with a mileage of about 120 thousand km, a 1.8T BU engine and an automatic transmission of 2008 Volkswagen cars. The car enters the factory to replace the oil maintenance. When the oil is changed, the throttle is cleaned and the vehicle is not started. The maintenance technician thinks that when the throttle is cleaned, too much of the detergent spray can not start, so the spark plug is dismantled and the compressed air is blown in the cylinder. Because this phenomenon is not free to remove the fuel injector, cleaning up the exhausted liquid after the engine automatically extinguished, also can not beat the car. The reason is that the cleaning agent dampened the spark plug and the short circuit could not get angry. Continue to start causing fuel brush cylinder, engine cylinder pressure drop, unable to start. After cleaning the cylinder and spark plug, the vehicle is still unable to start. When the technical manager arrived at the scene, he found that the vehicle was running very light and the sound of running was different from that of the cylinder with low pressure. After consulting with FAW Volkswagen's maintenance station, it is known that the sudden top valve without any symptoms is a common problem. The engine was found to be bent at the top of the engine, and the top of the piston of the intake valve rocker was broken three, and the upper part of the piston was slightly damaged. Check the gas gate without cracks and replace the new valve. Grind the valve to test the valve without leakage. Check the displacement of the intake camshaft peach tip relative to the cam shaft without angle. Because the exhaust valve is not damaged, the assembly of the cylinder head is decided. The problem is not found in the timing chain. Replace the improved chain tensioner, install cylinder head and timing chain mechanism, start the car, do not find abnormal sound, there is no fault storage, after 5h operation. After one day, the customer found the vehicle special oil, originally a box of oil is expected to drive 800km, the actual driving 750km, after the maintenance is expected to drive 550km, the actual driving 500km, the vehicle computer shows an average fuel consumption of 100 kilometers 13L, the average fuel consumption before maintenance 8.7L. Inspection air flow meter in the case of no load, the intake amount of 3.2g/s, 0.5 to 0.8g/s higher than other miles, cleaning air flow sensor, not to solve the problem of high oil consumption. Check the oxygen sensor 33 sets of data blocks, keep the 0.45V unchanged, check the 99 sets of data blocks, show that the lambda adjustment is closed. Read the engine fault storage, no fault code, indicating that the closure is not caused by the failure, may be unattended, read 30 sets of data, show that the oxygen sensor is not ready, after the hot car runs 10min, 33 sets of data display 0.2 ~ 0.66V, about 7 times a minute jump. Measure tail gas, and all indexes are in the best value range. The reason why the maintenance technician gets the fuel cost of the engine is that the oxygen sensor always shows that the mixture is thin, and the frequency change is too slow, which indicates that the oxygen sensor is aging. For the sake of caution, we also switched to a different car, compared with the oxygen sensor value, and found that there was no big difference between the car and the car, so the oxygen sensor did not hold oil. And the customer response is to charge the fuel after the valve is finished, so the problem must be on the maintenance. Maybe the fuel injector and fuel rail leaks, but the engine can't smell gasoline. The intake port valve may be stuck, and no solenoid valve or vacuum line is found. The high pressure oil pump may leak into the engine, but the oil level does not rise. The piston that may be damaged will affect the cylinder pressure and the cylinder pressure 1100kPa. It may be that the deformation of the top of the piston changes the turbulent flow of the mixture, but it is not necessary to replace the piston when consulting the maintenance station. Maybe incorrect time, reading 91 sets of data, target timing angle 34 degrees, actual timing angle 33.5 degrees, basically set 94 groups, positive angle can display negative 20 to 34 degrees, the instrument display OK, the problem becomes very complex. Based on customer description, when the fuel consumption begins to occur at the top valve, the diagnostic range is reduced to the timing valve, and some elements of the valve may be overstressed. There is no problem to check the length of the timing chain, but the camshaft top has broken the roller rocker arm, indicating that the timing chain has been greatly stretched, so it decides to change the new timing chain. During installation, it is found that the new chain is tighter than the old one. A conclusion is drawn that the damage of the chain can not be judged by the length method. Check the intake cam shaft again, find that the positive wheel has a tooth angle displacement relative to the shaft, and the positive wheel has not found the wear of the positive wheel lock pin and the pin is not worn, but through the theory, it is known that the shape of the cam is very important and can not have a bit of displacement relative to the crankshaft, but it can not buy the exact same bump. Spare parts suppliers recommend using 2.0T camshaft. The car is normal when the axle is replaced.