Type of work: auto mechanic

Topic: Troubleshooting of the malfunction light of Lexus LS400 car and alarm detection method of oxygen sensor system.

Name: Qian

School: Xi 'an North Automobile Maintenance Vocational Training School.

Date: 65438+February 3, 2009

Troubleshooting of malfunction light and alarm detection method of oxygen sensor system on Lexus LS400 car

Author: money

Time: 65438+February 3, 2009

This paper mainly introduces a Lexus LS400 sedan of 1999. During driving, the engine fault indicator in the instrument lights up. When the instrument reads the fault code as 25 or 26(25 stands for the proportion of lean mixture and 26 stands for the proportion of rich mixture), it is known that the car lights up again during driving after maintenance, which means that it cannot be completely repaired according to the fault code.

Keywords: fault code; Oil supply system; λ sensor

Automobile electronically controlled fuel injection engine is the product of mechatronics, especially the control system of the engine, which is equipped with multiple sensors, actuators and electronic control elements. When the control system works, various signals cross each other to control air intake, fuel injection and ignition. Once a fault occurs, the boundaries of symptoms are blurred. In addition, if one system fails, the computer control will display the failure code of another system. Therefore, we must fully understand the structural principle of the electronically controlled fuel injection engine, master the related functions, and make a feasible maintenance plan by using scientific analysis methods and maintenance skills.

main body

Fault symptoms: a Lexus LS400(UCF 10 engine) car, the engine fault light is on, and the fault code read is 25 or 26.

Troubleshooting: According to the data, it is known that the oil supply system is faulty (25 stands for lean mixture and 26 stands for rich mixture). Under normal circumstances, if the fault code is read as 25 or 26, you will know that it is a fault of the oil supply system. Then the next step is to check the oil circuit, that is, check the ignition components such as spark plugs and high-voltage lines, replace the gasoline filter, and clean the fuel injectors. The purpose of this is to ensure the normal ignition of the engine, smooth oil supply and correct fuel injection. After these tasks are completed, the fault code is eliminated and the fault light goes out. However, about 200Km after the vehicle was repaired, the engine fault light came on again, and the fault code read after returning to the factory for repair was still 25 or 26. There should be no problem with the oil supply system, but why? We carefully searched the components related to ignition and fuel supply, and found that the voltage fluctuation value of the oxygen sensor obviously did not meet the specified requirements (standard: when the output voltage is lower than 0.35V or higher than 0.7V, it will jump more than 4 times in 10S). After replacing the oxygen sensor, the fault light will no longer light up.

Fault analysis: Why does fault code 25 or 26 indicate that the oxygen sensor is not working properly, but fault code 2 1, 27 or 28 of the oxygen sensor is not displayed? According to the analysis of the working principle of fuel injection, it can be seen that the length of fuel injection time is corrected by the computer according to the output signals provided by each control element. Because the oxygen sensor is not working normally (not completely failed), that is, the output voltage value does not meet the specified requirements, the computer gives the injector the wrong fuel injection pulse width after getting the incorrect voltage signal from the oxygen sensor, resulting in the fuel injection quantity.

Too little or too much, that is, the mixing ratio is too thin or too thick. When the actual number of faults is accumulated, the computer will form fault memory, which is why the fault light will light up again after the maintenance is about 200km.

This kind of fault gives us an inspiration, that is, when the fault light of Lexus LS400 engine is on and the fault code is 25 or 26, we should first check whether the oxygen sensor is normal. If it is lower than the specified voltage value, we must replace it, and then check the oil circuit, so as to completely eliminate the fault.

Summary: This situation is just the opposite, that is, the oxygen sensor itself is not faulty. In electronically controlled gasoline injection engine, oxygen sensor is an electrical component used for closed-loop control of fuel system. It is mainly used to measure the oxygen content in exhaust gas and feed the measured data back to ECU in the form of voltage signal to control the air-fuel ratio of the engine to 14.7. At the same time, it is also an alarm element for various fault signals.

Zirconia sensor is a common oxygen sensor, and its faults are mostly manifested in that the surface is covered with lead compounds or carbides, which makes the gas impermeable and oxygen ions unable to diffuse, leading to failure. When the fault light gives an alarm and reads the sensor fault code, it must be diagnosed. However, the alarm of the oxygen sensor system does not necessarily mean that the sensor is faulty. The alarm signal is also affected by the following factors: ① the working condition of the ignition system; (2) Sealing performance of inlet system; ③ Whether the exhaust system is blocked; (4) the working condition of the syringe; ⑤ The oil pressure in the oil supply system is high and low.

Fault diagnosis of 1. oxygen sensor

According to the characteristics of zirconia sensor, when the air-fuel ratio is maintained at 14.7, the reference voltage of alarm signal is 0.4-0.5V；; When the air-fuel ratio is greater than 14.7, its voltage rises to 0.8- 1V, indicating that the mixture is too rich. When the air-fuel ratio is greater than 14.7, the voltage drops to about 0.2V, indicating that the mixture is too lean.

The method for diagnosing the working condition of the oxygen sensor is as follows:

(1) Keep the engine speed at about 2500 rpm and preheat the sensor for 2 minutes.

(2) Unplug the sensor plug (pay attention to the pin position of the sensor with heating coil), measure the feedback voltage with a multimeter, and check the number of times the voltmeter pointer swings within 10S; (1) If the pointer of the voltmeter swings less than 8 times, preheat the sensor again and check the number of pointer swings every 10 second. At this time, if the pointer swings more than 8 times, it means that the oxygen sensor works normally; (2) If it is still less than 8 times, disconnect the sensor harness plug and measure its feedback voltage again; When the voltage is greater than 4.5V, disconnect the vacuum tube on the intake pipe. At this time, if the pressure is still greater than 0.45V, the sensor is damaged. If the voltage is lower than 0.45V, the mixture is too rich, and the oil inlet or control system should be checked. When the voltage is lower than 0.45V, unplug the water temperature sensor and connect a 4-8 K Ω resistor. At this time, if the voltage is still below 0.45V, the sensor is damaged. If it is greater than 0.45 volts, it indicates that the mixture is too lean.

2. Detection of the working state of the ignition system

Firstly, the microcomputer-controlled ignition system is routinely inspected, including the working conditions of spark plugs and high-voltage lines, spark energy, ignition timing, ignition advance angle and so on. The ignition method is: connect the red clip of the timing lamp to the battery sensor and the cylinder of a high-voltage line, and the ignition timing lamp is aimed at the ignition timing mark on the front pulley of the engine. When the engine speed increases, the ignition advance angle should increase. At this time, knock the knock sensor fixing screw or cylinder head with a hammer or wrench, and the ignition advance angle should be obviously delayed.

3. Air intake system sealing performance inspection

Connect the vacuum gauge to the intake pipe. When the engine is idling, the vacuum degree of the intake pipe should be within the range of 57.33-70.66kpa, otherwise the intake system will leak. If the pointer of the vacuum gauge gradually returns to zero, the exhaust system is blocked.

4. Fuel injector performance inspection

The fuel injection quantity of the fuel injector depends on the fuel injection pulse width, and when the pulse width is constant, it depends on the nozzle section and fuel injection pressure. The fuel injection quantity, atomization performance and sealing performance of the fuel injector were tested on the fuel injector test bench. Its main performance parameters are as follows: fuel injection duration 2ms, needle valve lift 0. 15mm, steady flow 2A, electromagnetic coil resistance 3-15 Ω, fuel injection amount 15s 45-55ml, and cylinder difference less than 5 ml.

5. Oil pressure detection of oil supply system

When the engine is running, connect the oil pressure gauge to the pressure tap of the fuel distribution pipe or the fuel pressure test point of the throttle body injection (TBI) to measure the oil pressure. Multi-point should be 200-350kpa, and single point should be 62-90 kpa. Or when the engine is working, clamp the oil return pipe, the oil pressure should rise by 100 kpa, and the engine speed should rise by 100r/min, indicating that the oil supply system is normal.

References:

Principle and detection of engine sensor: Liaoning Science and Technology Press: edited by Zhang Wei.

Electronic Control Vehicle Maintenance Data Manual: Heilongjiang Science and Technology Press: Editor-in-Chief: Zhang Yuexiang

Zhao Yingjun