Failure phenomenon: a 2002 year of Buick racing more than 200 thousand kilometers, the owner described: the car idling intermittently (5 ~ 6S periodicity) engine idle speed first reduced to about 700r/min, and then up to about 1200r/min, and then to the phenomenon of 900r/min. Fault diagnosis: connect the scanning tool TECH-2 on the diagnostic interface to retrieve the fault code, and there is no fault code stored in the engine computer PCM. Reenter the engine data list and view the data flow. It can be seen from the engine speed that the engine speed fluctuates in a larger range and circulates again and again. The main sensor data such as the idle speed step motor (IAC), the intake manifold absolute pressure sensor (MAP), the throttle position sensor (TPS) and other main sensor data have not been found. So after trying to disconnect IAC or MAP, the engine speed no longer reverberate, the engine fault indicator light in the instrument disc is lit (at this time the engine ECM detects the important sensor to lose the signal, ECM adopts the protection mode to make the engine stable at certain set speed and light fault indicator). When the oxygen sensor is disconnected, the time interval between failures is lengthened, but the symptom of the fault has not changed at all. Then our staff carried out the test of engine high pressure jump fire, the high pressure spark was normal, the spark plug was in good condition, and the possibility of the ignition system failure was eliminated. Then the oil pressure of the fuel system, the pulse width of the injector and the uniformity of each injector are tested. The result is still all normal, and the possibility of the fault in the oil system is eliminated. The circuit and the oil circuit system are normal, and there are no faults among the main sensors. What causes the failure? Our maintenance personnel are in deep thought. At this point, I think, if the engine exhaust system has problems, such as the exhaust is not smooth, the exhaust back pressure is too high and unstable is also the main cause of engine idle jitter, bad work. So we checked the exhaust manifold, the three way catalytic converter and the muffler, and found no abnormal phenomena. In order to eliminate the blockage and unreasonable structure of the exhaust pipe. We replaced a new set of exhaust pipes for testing, and the symptoms persisted. At this time the maintenance workers were lost. The camshaft of the racing car is driven by the crankshaft by the timing belt, and the car has been running over 60 thousand km, and the belt has not been replaced, and the possibility of failure in the valve timing can not be ruled out. Therefore, the timing system of the vehicle is checked, the timing mark is correct, and the timing belt is not damaged. At this point, the oil circuit, the circuit and the timing system are all right. The sensors in each part of the engine work normally. When restoring the timing system of the original vehicle, our maintenance personnel will shine before installing the crankshaft pulley. Is there a failure of the crankshaft pulley? If it fails, it will give ECM a wrong position of the crankshaft, causing the engine to work abnormal. So we carefully observed the crankshaft pulley of the car, and compared with the new crankshaft pulley that the crankshaft pulley was defective so that the signal sent by the crankshaft position sensor was disorderly and that the ignition signal of the ECM control was misplaced with the mechanical timing. The actual ignition advance angle is erroneous, causing the vehicle to shake seriously.