A Volkswagen Polaris with a mileage of about 15100km and equipped with a 1.6L engine. The car. After receiving the car: first use the V.A.G1552 fault detector to read the fault code, and the trouble free code is displayed. The main cause of idling jitter is the air leakage of the intake system; the oil path pressure is too low; the air filter is blocked; the injector atomization is bad, the oil leakage or blockage; the idle speed adjustment is unsuitable; the throttle body is bad; the spark plug work is bad; the air flow meter has malfunction; the cylinder pressure is too low and so on. This car is only in the idle condition engine jitter, when the speed increases the engine work normally, this means that the air flow meter has no fault, the cause of the failure is likely to be in the idle control valve, or the air intake system leakage phenomenon. Based on the above analysis, we have carried out the following checks: (1) check the wires between idle control valve and engine control unit to connect normally. Using V.A.G1552 fault detector to reset idle speed, the fault phenomenon still remains, indicating that the cause of the failure is not idle speed control valve. Check the throttle body and find that there is a lot of carbon deposition inside the throttle valve. The throttle channel is cleaned and installed, and the throttle body is basically set. After trial run, the idling speed has improved, but the jitter is still very serious. In the case of idle running, the carburetor cleaning agent is ejected to the connecting pipe in the intake system, and the engine speed does not change, indicating that there is no gas leakage in the intake system. 4. Connect the oil pressure gauge, start the engine, check the pressure in the fuel line at idle, meet the requirements. Check the spark plugs to jump fire, without exception. The pressure of each cylinder is measured within normal range. Is it a problem with the engine control unit? The replacement of an engine control unit failed. At the time of confusion, I accidentally touched the intake manifold and found that the intake manifold temperature was very high and felt hot. Normally, the intake manifold should not be hot. Can it be that the EGR valve (exhaust gas recirculation valve) has leaked the exhaust gas into the intake manifold? Check the EGR valve body, the temperature is very high, initially identified as EGR valve lax. Troubleshooting: replace 1 new EGR valves, test again, and troubleshooting. Fault analysis: the main fault of the engine is EGR valve leakage except for excessive volume of throttle valve. Under normal conditions, the EGR valve is opened only when the engine speed is high or medium and high. When the EGR valve is opened, the exhaust gas in the exhaust pipe is introduced into the fresh mixture to enter the combustion chamber, reducing the maximum temperature of the combustion chamber and reducing the emission of the harmful gas, but at the engine's heavy load and idle speed. The EGR valve should be closed to prevent exhaust gas from entering the intake manifold. If the EGR valve is closed, the exhaust gas in the exhaust pipe will enter the intake manifold at idle speed, causing the mixture to dilute and causing the engine to shake at idle speed; when the fuel injection pulse width of the injector increases, even if the EGR valve is closed, the exhaust gas from the exhaust pipe into the intake manifold will not affect the concentration of the mixture too much. So the engine does not shake after the speed rises.