How to solve the obvious jitter of 1.4t Passat?

Hello! Most of them are the problem of carbon deposition in the throttle, and there is an ignition system for reference! There are many and complicated reasons for poor idle speed of EFI engine, which increases the difficulty of fault diagnosis and elimination. This paper briefly analyzes the main causes of poor idle speed of L-type EFI system engine for reference of automobile maintenance personnel. 1 Idle switch signal circuit Cause The engine control computer (ECU) judges whether the engine is idling according to the potential of the idle switch signal (IDL terminal). When the idle contact is closed and the low potential is input to the IDL terminal of ECU, ECU judges that the engine is in idle working state, so it starts the idle control program to control the engine operation. Due to improper adjustment of idle contact clearance, poor contact, damage and circuit failure, engine ECU will not be able to correctly judge idle working conditions, resulting in idle control errors and various idle failures. Therefore, attention should be paid to the inspection, and this possibility should be ruled out first. 2 Idle control valve and its circuit reasons Idle control valve (ISC) is used to control the amount of bypass air that bypasses the throttle and enters the intake manifold at idle speed, thus controlling idle speed. According to the signal of water temperature sensor (THW terminal) and the switch signals of auxiliary devices such as air conditioner (A/C) and engine power steering oil pump, the engine ECU controls the engine speed to run stably at the set target speed, and the control process adopts the form of feedback control. ISC control valves are divided into stepper motor type, rotary solenoid valve type, duty ratio control type and vacuum solenoid valve type. When the ISC valve is jammed due to carbon deposit, and the control circuit is short-circuited, disconnected and grounded, the engine ECU can't correctly control the opening of the ISC valve, resulting in poor idle speed. Therefore, the diagnosis should focus on detection. 3 Air flowmeter and its circuit reasons Air flowmeter detects the amount of air entering the engine, which is one of the main bases for ECU to control fuel injection. The failure of the air flowmeter and its circuit makes the ECU unable to receive the air flow signal or the received signal distorted, which leads to inaccurate fuel injection by the injector, too rich or too thin mixture, resulting in low speed, fire or soft idle. During diagnosis, the digital multimeter can be used to detect the voltage at the output end of air flow signal and the corresponding input end of ECU at idle speed and compare it with the standard value. 4 Fuel injector and its circuit causes Fuel injector and its circuit faults affect the quantity and quality of fuel injection. For example, the fuel injection quantity is reduced and the atomization is poor due to the blockage of carbon deposit in the fuel injector, the wear of the nozzle hole leads to excessive fuel injection and oil leakage, and the electrical faults (poor contact, short circuit, open circuit and grounding) of the electromagnetic coil of the fuel injector lead to the reduction of fuel injection quantity and no fuel injection, resulting in soft idling and no ignition. 5 When the coolant temperature sensor and its circuit are idling, the engine ECU judges the thermal state of the engine according to the input signal (THW terminal) of the coolant temperature sensor and corrects the fuel injection quantity. When the water temperature is low, gasoline is difficult to evaporate, and the formation of mixed gas is difficult and uneven. Therefore, at low temperature, the fuel injection amount is appropriately increased and the mixture becomes rich. The bad water temperature sensor distorts the output signal, and ECU gets the wrong signal from THW terminal, resulting in improper correction. When the circuit is short-circuited or open-circuited, the computer uses limping control and fixes the water temperature of 80 degrees to control the idle speed, which often leads to low idle speed, misfire and soft operation. 6 The fuel pump and oil system cause the fuel pump and oil system to affect the fuel pressure. For example, if the pressure is too low, the actual fuel injection quantity of the injector coil will decrease, the spray quality will become worse and the idle mixture will become thinner under the same power-on time; If the pressure is too high, there will be too much fuel injection and too rich mixture. The pressure of fuel system is related to the technical conditions of fuel pressure regulator, fuel pump and oil pressure solenoid valve and their circuit working conditions. 7 air conditioning switch signal circuit causes the air conditioning (A/C) signal is a switch signal, which sends an air conditioning switch request to the computer. When the air conditioner is turned on, the computer will increase the idle speed in time according to the A/C signal to adapt to the load of the air conditioner compressor. If the A/C signal is abnormal, the idle speed will be too high or too low, and the engine will shake and stall. 8 Exhaust gas recirculation valve and its circuit reasons The exhaust gas recirculation valve (EGR valve) is only opened when the engine is at normal operating temperature and reaches a certain speed, and a part of exhaust gas is introduced into the intake manifold and returned to the cylinder to reduce the highest combustion temperature in the cylinder and reduce the emission of nitrogen oxides. The EGR valve is stuck in the open position, or it is not closed tightly when idling, or the circuit fault causes the idling to open, diluting the idling mixture, resulting in too low idling, soft operation and flameout. 9 Reasons for Neutral Start Switch Circuit For cars equipped with automatic transmission, ECU increases the idle speed according to the signal of neutral start switch, and automatically increases the idle speed when the transmission control lever is in reverse or forward gear, otherwise it reduces the speed. The neutral start switch circuit is faulty, and the ECU receives an error signal, which makes the idle speed too high or too low. 10 The ignition coil, igniter or ignition ECU, distributor, ignition signal generator, related sensors affecting ignition timing, high-voltage lines, etc. in the fault ignition system cause fire, weak spark and inaccurate ignition timing, resulting in poor idle speed. 1 1 Other faults In addition to the above fault causes, the following faults will also cause some idling anomalies: ECU fault; Main oxygen sensor circuit; Main relay circuit of EFI; Standby power supply circuit; Cold start injector circuit; The resistance value of that variable resistor is adjust by mixed gas; Fuel quality; The intake pipe leaks vacuum; Air filter is blocked; Bad cylinder compression, etc. In a word, there are many reasons for the poor idle running of EFI engine. During maintenance and diagnosis, comprehensive analysis should be made according to the specific symptoms, combined with the composition and structural type of EFI system, and the fault code should be generated by computer, and detected and judged by comparing with relevant technical data and materials. If the fault code is not read out, start with the possible faults that are easy to appear and eliminate, and diagnose step by step, so as to find out the reasons and eliminate the faults efficiently and accurately.