Understand and learn the choice and application of pressure sensors
Oxygen sensors are an essential component in engines that use three-way catalytic converters to reduce exhaust pollution. Since the air-fuel ratio of the mixture deviates from the theoretical air-fuel ratio, the purification ability of the three-way catalyst for CO, HC, and NOx will drop sharply. Therefore, an oxygen sensor is installed in the exhaust pipe to detect the concentration of oxygen in the exhaust gas, and The ECU sends a feedback signal, and then the ECU controls the increase and decrease of the fuel injection amount of the injector to control the air-fuel ratio of the mixture to be near the theoretical value.
At present, the actual application of the oxygen sensor is zirconia type oxygen sensor and titanium oxide type oxygen sensor. The common oxygen sensor has a single lead, two lead and three lead; a single lead is a zirconia oxygen sensor; a double lead is a titanium oxide oxygen sensor; and a three lead is a heated zirconia oxygen sensor. In principle, three lead-type oxygen sensors are not used instead.
If the oxygen sensor fails, the computer of the electronic fuel injection system will not be able to obtain the information of the oxygen concentration in the exhaust pipe, so the air-fuel ratio cannot be feedback-controlled, which will increase the engine fuel consumption and exhaust pollution, and the engine will be unstable. Lack of fire, surge and other phenomena. Therefore, it is necessary to troubleshoot or replace it in time.
First, the common fault of the oxygen sensor
1. Oxygen sensor poisoning
Oxygen sensor poisoning is a type of failure that often occurs and is difficult to control, especially for cars that often use leaded gasoline. Even new oxygen sensors can only work for thousands of kilometers. If it is only a slight lead poisoning, then using a box of lead-free gasoline, the lead on the surface of the oxygen sensor can be eliminated and returned to normal operation. However, due to the excessive exhaust gas temperature, lead is intruded into the interior, hindering the diffusion of oxygen ions, and the oxygen sensor is disabled, and then it can only be replaced.
In addition, silicon poisoning in oxygen sensors is also a common occurrence. In general, silica produced by burning silicon compounds contained in gasoline and lubricating oil, and silicone rubber gaskets using improperly emitted silicone gas will cause the oxygen sensor to fail, so use good quality fuel and lubricating oil. . The rubber gasket should be properly selected and installed during repair. Do not apply solvents and anti-blocking agents other than those specified by the manufacturer.
2. Carbon deposit
Because the engine is not burning well, carbon deposits on the surface of the oxygen sensor, or deposits such as oil or dust inside the oxygen sensor will block or block the outside air from entering the oxygen sensor, causing the signal output from the oxygen sensor to be out of alignment, and the ECU cannot be timely. Correct the air-fuel ratio. The accumulation of carbon is mainly caused by an increase in fuel consumption and a significant increase in emission concentration. At this point, if the sediment is removed, normal operation will resume.
3. Oxygen sensor ceramic chipping
Oxygen sensor ceramics are hard and brittle, and they can be broken and failed by hard objects or by blowing with a strong air stream. Therefore, be careful when handling and find problems in time.
4. Heater resistance wire is blown
For a heated oxygen sensor, if the heater wire is ablated, it is difficult to cause the sensor to reach its normal operating temperature and lose its effect.
5. The internal circuit of the oxygen sensor is broken.
Second, the oxygen sensor inspection method
1. Check of oxygen sensor heater resistance
Unplug the oxygen sensor harness connector and measure the resistance between the heater post and the grounding post in the oxygen sensor terminal with a multimeter resistance file. The resistance is 4-40Ω (refer to the specific model manual). If the standard is not met, the oxygen sensor should be replaced.
2. Measurement of oxygen sensor feedback voltage
When measuring the feedback voltage of the oxygen sensor, the harness connector of the oxygen sensor should be unplugged. According to the circuit diagram of the model, a thin wire is drawn from the feedback voltage output terminal of the oxygen sensor, and then the wire harness plug is inserted, and the engine is pulled out during operation. The feedback voltage is measured online (some models can also measure the feedback voltage of the oxygen sensor from the fault detection socket. For example, a series of cars produced by Toyota Motor Corporation can directly measure the oxygen sensor from the OX1 or OX2 terminals in the fault detection socket. Feedback voltage).
When detecting the feedback voltage of the oxygen sensor, it is preferable to use a pointer type multimeter having a low range (usually 2V) and a high impedance (internal resistance greater than 10MΩ). The specific detection methods are as follows:
1) Run the engine hot car to the normal working temperature (or 2min at 2500r/min after starting);
2) Connect the negative meter of the multimeter's voltage file to the E1 or battery negative in the fault detection socket, connect the OX1 or OX2 jack in the fault detection socket, or the number on the oxygen sensor harness connector to the outlet;
3) Let the engine keep running at about 2500r/min, and check whether the voltmeter pointer can swing back and forth between 0-1V, and record the number of times the voltmeter pointer swings within 10s. Under normal conditions, as the feedback control proceeds, the feedback voltage of the oxygen sensor will change continuously around 0.45V, and the number of changes in the feedback voltage within 10s should be no less than 8 times. If it is less than 8 times, it means that the oxygen sensor or feedback control system is not working properly. The reason may be that there is carbon deposit on the surface of the oxygen sensor, which reduces the sensitivity. In this regard, the engine should be run at 2500r/min for about 2min to remove carbon deposits from the surface of the oxygen sensor and then check the feedback voltage. If the voltmeter pointer change is still slow after clearing the carbon deposit, the oxygen sensor is damaged, or the computer feedback control circuit is faulty.
4) Check the oxygen sensor for damage
Unplug the oxygen sensor's harness connector so that the oxygen sensor is no longer connected to the computer and the feedback control system is in open loop control. Connect the positive pen of the multimeter's voltage file directly to the oxygen sensor feedback voltage output terminal, and the negative test lead is well grounded. When measuring the feedback voltage during engine operation, first disengage the crankcase forced air pipe or other vacuum hose connected to the intake pipe, artificially form a lean mixture, while watching the voltmeter, the pointer reading should be lowered. Then connect the disconnected pipe, then pull off the water temperature sensor connector, replace the water temperature sensor with a 4-8KΩ resistor, artificially form a rich mixture, and watch the voltmeter, the pointer reading should rise. It is also possible to change the concentration of the mixture by suddenly pressing or releasing the accelerator pedal. When the accelerator pedal is suddenly depressed, the mixture becomes thicker and the feedback voltage should rise. When the accelerator pedal is suddenly released, the mixture becomes thinner. The feedback voltage should drop. If the feedback voltage of the oxygen sensor does not change as described above, it indicates that the oxygen sensor is damaged.
In addition, when the titanium oxide type oxygen sensor is detected by the above method, if it is a good oxygen sensor, the voltage at the output end should fluctuate up and down around 2.5V. Otherwise, the sensor can be removed and exposed to air, and its resistance value measured after cooling. If the resistance value is large, the sensor is good, otherwise the sensor should be replaced.
5) Inspection of the appearance color of the oxygen sensor
Remove the oxygen sensor from the exhaust pipe and check the vent hole on the sensor casing for blockage and damage to the ceramic core. If it is damaged, the oxygen sensor should be replaced.
The fault can also be judged by observing the color of the top part of the oxygen sensor:
1 light gray tip: this is the normal color of the oxygen sensor;
2 white top: caused by silicon pollution, the oxygen sensor must be replaced at this time;
3 brown top: caused by lead pollution, if serious, must also replace the oxygen sensor;
4 Black top: It is caused by carbon deposit. After removing the engine carbon deposit fault, the carbon deposit on the oxygen sensor can be automatically removed.