202 10 solar terms
1. Idle switch is not turned off.
Fault analysis: When the idle contact is disconnected, ECU determines that the engine is in a partial load state. At this time, ECU determines the fuel injection quantity and time according to the air flow sensor, crankshaft position and speed signal. At this time, the engine is idling, and the intake air is less, which leads to the mixture being too rich and the speed increasing. When the ECU receives the signal that the mixture is too rich from the oxygen sensor, it will reduce the fuel injection and increase the opening of the idle control valve, which will lead to the mixture being too lean and slow down the vehicle speed. When the ECU receives the signal that the mixture fed back by the oxygen sensor is too lean, it will increase the fuel injection and reduce the opening of the idle control valve to make the mixture too rich, thus increasing the speed. This repetition makes the idle speed of the engine unstable. When idling, turning on the air conditioner, steering wheel and turning on the light will increase the load of the engine. In order to prevent the engine from stalling due to increased load, ECU will increase the fuel supply to maintain the balanced operation of the engine. When the idle contact is disconnected, ECU thinks that the engine is not in idle state and will not increase the fuel supply, so the speed will not increase.
Diagnostic method: Turn on the air conditioner and turn the steering wheel when idling. If the engine idle speed does not increase, it is proved that the idle speed switch is not closed.
Troubleshooting: Adjust or replace the throttle position sensor.
2. Idle speed control valve is faulty.
Fault analysis: The normal idle speed of EFI engine is guaranteed by idle speed control valve (ISC). According to the signals of engine speed, temperature, throttle switch and air conditioner switch, ECU increases the intake bypass channel of idle speed control valve or directly increases the throttle opening to increase the intake air and improve the engine idle speed. When the idle speed is higher than the set speed, ECU instructs the idle speed control valve to close the air intake bypass channel to reduce the air intake and the engine speed. Oil pollution and carbon deposition lead to the action card of idle speed control valve being issued or the throttle valve not being closed correctly, which will make ECU unable to adjust the engine idle speed correctly and lead to unstable idle speed.
Diagnostic method: Check the action sound of idle speed control valve. If there is no action sound, the idle speed control valve is faulty.
Troubleshooting: Clean or replace the idle speed control valve, and basically set the idle speed with a special decoder.
3. Air leakage in the intake pipe
Fault analysis: According to the idling control principle of the engine, in general, the opening of the idling control valve strictly follows a certain functional relationship with the air intake, that is, the opening of the idling control valve increases, and the air intake increases accordingly. The air leakage in the intake pipe makes the intake volume and the opening of the idle speed control valve not strictly follow the original functional relationship, and the air flow sensor cannot measure the real intake volume, which leads to the inaccurate control of the intake volume by ECU and the unstable idle speed of the engine.
Diagnosis method: If you hear the air leakage sound of the air intake pipe, it proves that the air intake system is leaking.
Troubleshooting: Find leaks, reseal or replace related parts.
4. Wrong valve timing.
Fault analysis: For vehicles using mass flow air flow sensor, the sensor uses constant temperature difference control circuit to detect air flow. Its control circuit is a bridge circuit composed of heating element, temperature cathode, precision resistor and sampling resistor. When air flows through the heating element to cool it, the temperature of the heating element decreases, the resistance decreases and the bridge voltage loses balance. The control circuit will increase the current supplied to the characteristic element to keep the temperature difference and temperature resistance constant. The magnitude of the current increment depends on the degree to which the characteristic element is cooled, that is, on the amount of air passing through the air flow sensor. When the current increases, the voltage on the sampling resistor will increase, thus converting the change of air flow into a voltage signal and outputting it to ECU, and ECU will set the basic fuel injection quantity according to this signal. Due to the valve timing error, the valve will not open and close at the specified time, which will reduce the amount of air entering the cylinder. At the same time, the temperature in the intake manifold will also increase due to blowby, which will reduce the cooling degree of natural components. Therefore, the voltage signal output to ECU will be low, and the fuel injection quantity will be reduced, which will easily lead to unstable engine operation and idle jitter.
For vehicles with D-type fuel injection system, the absolute pressure sensor of the intake manifold converts the pressure (⊿Px) signal of the intake manifold into a voltage signal and outputs it to the ECU, and the ECU issues instructions to make the fuel injectors inject fuel. Therefore, ⊿Px is the basis for ECU to determine the fuel injection quantity. When the valve timing is wrong, ⊿Px will fluctuate beyond the standard, which will lead to the fluctuation of fuel injection quantity and make the engine idling unstable.
Diagnostic method: Check cylinder pressure, ⊿Px and timing mark. If the cylinder pressure or ⊿Px is not within the standard value range and the timing mark is incorrect, it can be judged that the valve timing error has occurred.
Troubleshooting: Check the timing mark and readjust the valve timing according to the standard.
5. The fuel injector leaks or is blocked.
Fault analysis: The fuel injector leaks oil or is blocked, which makes it impossible to inject fuel according to ECU's instructions, thus causing the mixture to be too rich or too thin, making individual cylinders work badly and leading to unstable engine idle speed. The lean mixture caused by the blockage of the fuel injector will also cause the oxygen sensor to generate a low potential signal. According to this signal, the electronic control unit will issue instructions to enrich the mixture. When the command exceeds the control limit, ECU will mistakenly think that the oxygen sensor is faulty and remember the fault code.