The Shanghai Volkswagen 1. 4L car with a mileage of about 90 thousand km in 2007. When the vehicle is cold, the idle speed is jitter, and the hot car is improved. Failure analysis: after asking the user, he learned that the car was a second-hand car he had just taken over, and the maintenance files were unknown. After taking over for a period of time, it is found that the engine will shake when it is idle, and there is a noticeable feeling inside the car. The maintenance personnel took the basic check after receiving the car. Using the fault diagnosis instrument V. A. G1552 test system, the cylinder pressure and the fuel pressure are basically normal, using endoscope, it can be seen that each cylinder inlet valve has different degrees of carbon accumulation. Because the Baltic 1. 4L engine easy to appear the intake valve charcoal, causing cold car valve closure is not strict, the cylinder pressure is not enough to make the cold car idle jitter, hot car after the carbon gelatin softened, the intake door can be normally closed and the cylinder pressure can be set up normally, the phenomenon of idle jitter will disappear. According to this idea, the maintenance personnel dismantled the intake manifold and cleaned up the carbon deposition and cleaned the injector. Then test the car, normal, so the car out of the station. After a few days, the vehicle came back to the station again. The user reflected the engine idle jitter fault again. Again, the engine was checked and the road test was repeated. The fault diagnosis instrument inquired the 1 accidental fault code 17622 (2 cylinder injector N31 to the positive electrode short circuit). It is observed that the engine occasionally shakes seriously when it is idle, but it accelerates normal. It is found that the brake is not sensitive in the road test, and it is hard when the brake pedal is stepped down. The vacuum degree of intake manifold is measured at idle speed, although it is somewhat low, but it is still in the normal range. The 2 cylinder injector circuit was checked and the results were all normal, but when the engine control unit was checked, the jitter vanished if the engine control unit was tapped when the idling jitter was tapped. So I judge the engine control unit internal control of the injector chip damage, the replacement engine control unit, the user reflects the engine idle speed and faster speed. But when I tried it, I felt that the engine was still slightly jitter and it was still hard when I stepped on the brake pedal. A summary of the previous maintenance process, the related circuit has been checked for 2 times, the possibility of the problem is very small, then the brake pedal hard fault is likely to be the intake manifold vacuum degree. In order to eliminate the fault thoroughly, the cylinder head is disassembled and the intake and exhaust doors, camshaft and the hydraulic tappet are carefully checked. Besides the small area of smooth wear marks on the top of the 2 cylinder flange of the intake camshaft, there are no other problems. Is smaller wear and tear affect the opening stroke of the inlet valve? Trouble shooting: because of the special shape of the camshaft, we can not measure it accurately, so the intake camshaft is replaced. After assembly, the phenomenon of engine idling slightly jitter disappeared. Road test, to determine the brake pedal is hard to troubleshoot. Failure summary: why does the smaller wear of the intake camshaft have such a big effect on the brake vacuum booster? The valve type of the 1. 4L engine is different from other engines using hydraulic tappet. Most engines are cams acting on hydraulic tappet and hydraulic tappet actuated valves. The Baltic 1. 4L engine has more than 1 leveraged rocker arms. Even if the camshaft is less worn, the impact of the lever can be magnified by the magnification of the lever. Thus, the opening stroke of the 2 cylinder inlet is affected, the engine idle jitter is caused, and the vacuum degree of the intake manifold is low, and the brake pedal will be hard when the brake is brake. In the entire troubleshooting process, the author also found that the inlet and exhaust camshaft marks on the real vehicle were not in conformity with the maintenance manual. In the maintenance manual, the intake camshaft sign is 036BE, and the exhaust camshaft sign is 036BD; on the real car, the intake camshaft sign is 36DG, and the exhaust camshaft sign is 36AA.