The idle speed of EFI engine is controlled by computer, which controls the engine idle speed according to water temperature signal, engine speed signal and air conditioning load signal. What if the idle speed is high? The following is the reason why the idle speed I sorted out for you is too high. I hope it is useful to you.
1. The idle switch is not closed
Fault analysis when the idle contact is disconnected, the ECU determines that the engine is under partial load. At this time, the ECU determines the fuel injection quantity and time according to the air flow sensor, crankshaft position and speed signals. At this time, the engine works at idle speed, with less intake, resulting in too rich mixture and rising speed. When ECU receives the signal that the mixture is too rich fed back by the oxygen sensor, it will reduce the fuel injection quantity and increase the opening of the idle control valve, which will cause the mixture to be too thin and reduce the speed; When the ECU receives the signal that the mixture is too lean fed back by the oxygen sensor, it will increase the fuel injection quantity and reduce the opening of the idle control valve, which will cause the mixture to be too rich and increase the speed. Such repetition makes the engine idle unstable. When idling, turning on the air conditioner, turning the steering wheel and turning on the headlamp will increase the load of the engine. In order to prevent the engine from stalling due to the increase of load, the ECU will increase the fuel supply to maintain the balanced operation of the engine. When the engine speed is in idle condition, the ECU will not increase the fuel supply, so the engine speed will not be disconnected.
Diagnostic method: turn on the air conditioner and turn the steering wheel at idle speed. If the idle speed of the engine does not increase, it proves that the idle switch is not closed.
Troubleshooting adjust or replace the throttle position sensor.
2. The 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 engine speed, temperature, throttle switch, air conditioning switch and other signals, the ECU will open the air inlet bypass channel for the idle control valve through calculation or directly increase the opening of the throttle to increase the air intake to improve the engine idle speed; When the idle speed is higher than the set speed, the ECU will command the idle control valve to turn down the air inlet bypass channel to reduce the air inlet and reduce the engine speed. The idle speed control valve caused by oil stain and carbon deposit is stuck or the throttle valve is not closed in place, which will make the ECU unable to adjust the engine idle speed correctly, resulting in unstable idle speed.
Diagnostic method: check the action sound of the idle control valve. If there is no action sound, the idle control valve is faulty.
Troubleshooting: clean or replace the idle speed control valve, and use the special decoder to set the idle speed basically.
3. Air leakage of intake pipe
Fault analysis can be seen from the idle control principle of the engine. Under normal circumstances, the opening of the idle control valve and the air intake strictly follow a certain functional relationship, that is, the opening of the idle control valve increases, and the air intake increases accordingly. The intake pipe leaks, so that the intake volume and the opening of the idle speed control valve do not strictly follow the original function relationship, and the air flow sensor cannot measure the real intake volume, resulting in the inaccurate control of the intake volume by the ECU, resulting in the unstable idle speed of the engine.
Diagnostic method: if you hear the "Chi Chi" sound of leakage in the intake pipe, it indicates that the intake system is leaking.
Troubleshooting: find the leakage, re seal or replace relevant components.
4. Valve timing error
Fault analysis for the vehicle model using mass flow air flow sensor, this sensor adopts constant temperature difference control circuit to detect the air flow. The control circuit is a bridge circuit composed of heating element, temperature cathode, precision resistance and sampling resistance. When the air flow 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 maintain a certain temperature difference with the temperature resistance. The magnitude of the current increment depends on the nature and the degree to which the 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 resistance will rise, so as to convert the change of air flow into a voltage signal output to the ECU. The ECU sets the basic fuel injection quantity according to this signal. The wrong valve timing will cause the valve not to open and close at the specified time, resulting in the reduction of the amount of air entering the cylinder. At the same time, due to the blowby, the temperature in the intake manifold will also increase, so as to reduce the cooling degree of the property components. Therefore, the voltage signal output to the ECU will be low, and the oil injection will be reduced, which is easy to cause the engine to run unsteadily and shake at idle speed.
For vehicles with D-type fuel injection system, the intake manifold absolute pressure sensor will the pressure of the intake manifold (⊿ p_) The signal is converted into a voltage signal and output to the ECU, which sends a command to make the fuel injector inject fuel. So ⊿ p_ It is the basis for ECU to determine the fuel injection quantity. Incorrect valve timing will cause ⊿ p_ It exceeds the standard and fluctuates, which causes the fluctuation of fuel injection quantity and makes the engine idle unstable.
Diagnostic method: check the cylinder pressure, ⊿ p_ And timing mark, if cylinder pressure or ⊿ p_ If it is not within the standard value range and the timing mark is incorrect, it can be judged that the valve distribution phase 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 or is blocked, which makes it unable to inject fuel according to the instruction of ECU, resulting in too rich or too thin mixture, poor operation of individual cylinders and unstable idle speed of the engine. The mixture caused by the blockage of the fuel injector is too thin, which will also cause the oxygen sensor to generate a low potential signal. The ECU will send a command to enrich the mixture according to this signal. When the command exceeds the regulation limit, the ECU will mistakenly believe that the oxygen sensor has a fault and memorize the fault code.
Diagnostic method: use a stethoscope to check whether the fuel injector makes a "creak creak" sound or measure the fuel injection quantity of the fuel injector. If the fuel injector has no action sound or the fuel injection quantity exceeds the standard, the fuel injector is faulty.
Troubleshooting: clean and check the fuel injection quantity of each fuel injector and confirm that there is no blockage and leakage.
6. The exhaust system is blocked
Fault analysis when the three-way catalytic converter is partially blocked due to carbon deposition, crushing and other reasons, it will increase the exhaust resistance and reduce the negative pressure of the intake pipe, resulting in poor exhaust and insufficient intake of the engine, resulting in poor engine performance and shaking at idle speed, which may also cause the ECU to remember the fault code of the air flow sensor. If the fault is not eliminated for a long time, the oxygen sensor will work under harsh conditions for a long time, accelerate the damage of the oxygen sensor, and cause the engine fault indicator to light up.
Diagnosis method: use vacuum gauge to detect the, if ⊿ p_ If it is low and the acceleration is often accompanied by a stuffy sound, it can be determined that the three-way catalytic converter is blocked.
Troubleshooting replace the three-way catalytic converter.
7. EGR valve is opened under idle condition
Fault analysis EGR valve can only be opened when the engine is under medium and small load. The function of EGR is to enter part of the exhaust gas into the combustion chamber, reduce the temperature in the combustion chamber and reduce No_ Emissions. However, too much exhaust gas participates in combustion, which will affect the ignition performance of the mixture, thus affecting the power performance of the engine, especially under the working conditions of engine idle speed, low speed and small load (at this time, ECU controls the exhaust gas not to participate in combustion to avoid affecting the engine performance). If the EGR valve is opened when the engine is idling, so that the exhaust gas enters the combustion chamber to participate in combustion, the combustion will become unstable and sometimes even misfire.
Diagnostic method: remove EGR valve. Block the exhaust gas recirculation channel and the fault disappears.
Troubleshooting this fault is mostly caused by the carbon deposit on the EGR valve stuck in the normally open position. The fault can be eliminated by eliminating the carbon deposit on the EGR valve or replacing the EGR valve.
Change the oil. When there is a problem with the idle speed of the vehicle, we should first see whether it is time to change the oil. If it is time to change the oil, it should be changed in time, otherwise the engine will be abnormal if the time is too long.
Attention should be paid to the selection of engine oil. Automobile engine oil should be of high grade, such as SJ grade or above. The quality of such engine oil is better and its performance is good in all aspects.
Look at the antifreeze tank. The antifreeze in the water tank should also be checked more, not less or too much, otherwise it will affect its effect and can not play a good role in cooling the engine.
Pay attention to antifreeze. Generally, new cars need to be replaced once every 2-3 years, or every 40000 kilometers. The antifreeze with proper freezing point shall be selected according to the temperature of the area. For example, if the ambient temperature is as low as - 10 °, choose the antifreeze with - 25 °.
Check the engine. If the idle speed of the vehicle is not the problem of oil and antifreeze, check the engine to see if there is a problem with the engine.
Check the oil circuit. Sometimes the idle speed of the vehicle is too high, which is likely due to oil circuit problems, such as throttle blockage and so on.