The Korean modern four cylinder engine manual transmission car stops in the urban area when it meets the traffic signal red light. Fault diagnosis: in the maintenance plant to repair the engine, check, adjust and repair the suspected fault parts, for example, the replacement of spark plug, high pressure line, cleaning nozzle, throttle body, cylinder cover, etc., all can not be solved, so the car is parked in the maintenance plant. The method of determining the cause and location of failure is nothing more than empirical method, causal analysis method, diagnostic technology and method. But the diagnostic method is most effective, simple and accurate. I asked the maintenance personnel what the trouble was and why I put it here. The maintenance personnel told the author that when the car was running, he met the red light and stopped the fire. Failure is an objective existence, but the phenomenon is subjective description, there is an inaccurate or incomplete description of the possible factors, and there is a lack of the initial conditions, boundary conditions and other necessary conditions for the failure. The author tried the test with the maintenance personnel. First, he wanted to confirm the condition of the malfunction and the phenomenon. Second, make a preliminary judgement of whether the part of the engine system is normal. The result of the observation is the engine's idle speed. The road test shows that the engine overtaking ability is normal, the maximum speed is normal, and the climbing ability is normal. The basic conclusion is that the diagnosis parameters of the three engines are normal, the air supply system is normal, the fuel supply system is normal, and the ignition system is normal. Thus, it can be determined that it is an idle speed control fault. Because there is no one-to-one correspondence between the fault phenomenon and the cause and location of the failure, it is not necessary to carry out the causal analysis. The best method of diagnosis is the system method. Diagnosis by system is simple and efficient, and the cause and location of faults can be determined accurately. The air supply system of Korean modern car engine adopts air flowmeter metering method, throttle body adopts direct control mode. That is, idle speed servo motor controls throttle opening to achieve idle speed feedback regulation. Through the road test method, it has been proved that the engine idle speed control system has failed. According to the structural characteristics of idle speed control and the grasp of idle speed control content, necessary measurements and tests are carried out. It is proved that the idle speed signal is normal and the idle speed control mode is normal (steering, headlamp, air conditioner), but the performance of the idle speed actuator is poor. Replace idle motor. Idle motors (auxiliary parts or refurbished parts) are bought and installed for trial run, resulting in failure. After the test came back, the procedures and specifications of the diagnosis process were reconsidered, proving that there is no problem in the diagnosis procedure and specification. As a result, the idle motor was changed to replace the original parts. After the replacement, the test run was normal and the trouble was eliminated. Maintenance summary: the diagnosis process is the concrete embodiment of diagnostic thinking. The diagnosis must be carried out according to the procedure so as to ensure the correctness of the diagnosis. The diagnosis is not standardized, as well as the correct diagnosis results. Believe in your own diagnostic procedures and procedures, and pay attention to the operation rules in the diagnosis process. The description of an objective engine should reflect its characteristics and structural differences, which is the precondition for correct analysis and diagnosis. There is no diagnostic value for the description of vehicle system, vehicle type, vehicle fare and kilometers traveled. Only when there are fewer cars in the past and the method of "Hope", "smell", "ask" and "cut" in traditional Chinese medicine is there a certain value.