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A Honda accord with a mileage of about 74 thousand km. Users reflect: the car starts difficult, accelerates shivering, idling is not stable, cold car phenomenon is more serious. The car's engine trouble lights are sometimes bright. Fault diagnosis: the inspection found that the car does not step on the accelerator pedal is more difficult to start, step down the accelerator pedal is easier to start, but after starting a lift engine will be flameout. If you start to step on the accelerator pedal after a period of time, the engine can run slowly after a period of time, but the engine can operate, but the idle speed is unstable, changes between 450 and 650r/min, the vacuum degree changes between 47 and 55Pa, accelerating to more than 2500r/min, everything is normal. This kind of fault often occurs on other vehicles. Most of the faults are caused by too dirty throttle or serious carbon deposition on idle control valves. When the cold car starts, the intake volume is more, although the computer will control the idle control valve to correct, but this requires a process, but this kind of fault will rarely cause the fault light to light. First of all, the air intake system is checked and cleaned. The inspection result is that all the pipelines of the intake system are connected well without leakage and blockage. The throttle position sensor and the idle speed control valve work well. Check the engine electronic control system with the fault diagnosis instrument. The read out fault code is P0131 - oxygen sensor circuit voltage is low. Remove oxygen sensor and no carbon deposit on the surface. The connection of the wires is reliable, indicating that the oxygen sensor is normal. However, the voltage is always less than 0.45V, indicating that the mixture is too thin. Pull down the wire harness joint of the water temperature sensor, and connect a rheostat with a value of 4 ~ 8K Omega. (because a correction signal of a water temperature sensor is controlled to reduce the amount of fuel at high temperature, increase the amount of fuel injection at low temperature, add a rheostat, and increase the amount of fuel at zero degree) again and find oxygen sensing again. The voltage of the device is close to 0.9V, which indicates that there is no problem with the oxygen sensor, but the intake is too thin. The fuel pressure is measured to be 285kPa, normal. After the fuel injector is removed, the fault remains unchanged and the fault code is only P0131. Pulling out other sensor tests, can read the corresponding fault code, indicating that ECU is no problem, guessing the fault phenomenon is caused by leakage. Check the intake system and the connected vacuum tube and find no abnormalities. Check the EVAP and EGR system. After pulling the vacuum tube on the EGR valve, the idle speed of the engine rises to 1000r/min, and the vacuum degree also rises and is stabilized at 68kPa. After starting, the vacuum tube on the EGR valve is removed, and the tube is blocked by hand. It feels vacuum suction. Under normal circumstances, there should be no vacuum. It is precisely because of the vacuum suction that the exhaust gas circulate at idle speed, so that the mixture is too thin and the idle speed is unstable. Remove the EGR control solenoid valve wiring harness plug, found that the vacuum tube is still vacuum, fully shows that EGR control solenoid valve failure. Further examination found that the EGR controlled the solenoid valve was dirty, so cleaned with the chemical cleaning agent and dripped two drops of clean oil. Maintenance summary: some fault codes are not the fault of the sensor itself. In this case, the oxygen sensor fault code appears in the case, which may be caused by the excess or excessive dilution of the mixed steam. The daily maintenance should not be replaced blindly, because many parts are not bad parts, and a lot of parts can be reused when they know their structural features. Review: there are many reasons for the engine idling instability. According to the three elements of the normal work of the engine, the analysis and inspection is the basic method to diagnose the idling instability. The author also gives a detailed introduction to this article. It is especially worth affirming that the test of vacuum gauge confirms that the idle speed is caused by the dilution of mixed steam. However, it can be seen from the article that in finding the true cause of the fault, a little curve is taken, such as testing the fuel pressure, checking and cleaning the nozzle, checking the oxygen sensor, simulating the temperature sensor signal of the coolant and adding the concentrated mixed steam. Of course, these factors also cause idling instability, but if the author grabs the engine faster than 2500r/min, all normal running features, and the analysis of the fuel correction value in the engine control, it will soon be possible to determine that the idling instability is caused by the intake leakage (leakage vacuum). When the fuel correction value is too large, the fuel correction value will decrease as the throttle opening increases. Because the engine idling operating conditions require a strong mixture of steam, if there are no measured air into the intake port behind the throttle, the air fuel ratio is increased, and the mixed steam is thinner and the normal operation of the idle speed is affected. With the increase of the engine load, the intake volume becomes larger and the leakage of the vacuum is negligible, so the engine's normal working load is normal. EGR valve card lives in open state, which is one of the common faults that cause idle speed instability. Usually there will be a fault phenomenon described in this paper.