Maintenance points of air flow sensor

The air flow signal is one of the main signals that the engine electronic control unit (ECU) controls the concentration of mixed steam. If the intake air volume increases, the injection quantity controlled by ECU is also large, and vice versa. Although the failure of the air flow sensor will not cause the engine to fail to start, it will definitely affect the dynamic performance of the engine, such as unstable idle speed, poor acceleration, "tempering" of the intake pipe, black smoke from the exhaust pipe, and excessive exhaust emissions.

(1) causes poor engine acceleration.

A Passat GLi car, with a mileage of 45000km, the engine accelerated to 4200 r/min, and then stepped on the accelerator, but the engine speed decreased. Tested with VAG 155 1 fault diagnosis instrument, and no fault codes are stored. Reading the data stream of 4200r/min, it is found that the air flow data can only reach1.1~1.3g/s, and cannot change with the opening and closing of the throttle. After replacing the air flow sensor, the fault was eliminated. Investigate the reason. The deviation of the output signal of the air flow sensor is not enough for the electronic control unit (ECU) to record the fault code. However, because the air flow signal can't accurately reflect the actual air intake, the ECU controls less fuel injection accordingly, so the engine speed drops instead of rising.

⑵ Causes the intake pipe to "backfire"

A Jetta King car. Engine idling jitter, rapid acceleration when the intake pipe "backfire". Check the air intake system, there is no air leakage. Replacing the fuel filter and cleaning four fuel injectors are ineffective. Check whether the fuel pressure is normal at idle speed and acceleration. When the connector of the air flow sensor was removed for debugging, the fault phenomenon was greatly improved. Measure the resistance value of each terminal of the air flow sensor, which is normal. Finally, it is found that dust is stuck on the hot film resistor of the air flow sensor. After cleaning the dust with carburetor cleaner, troubleshoot.

For EFI engine with hot film air flow sensor, a constant voltage is applied across the hot film (resistor) to raise the temperature of the resistor, and its temperature is controlled by the circuit. According to the current flowing through the hot film resistor, ECU judges the air intake and determines the fuel injection quantity to meet the needs of different engine working conditions. If dust accumulates on the hot film to form a thermal insulation layer, when the air intake increases, its temperature changes slowly, the required current decreases, and the fuel injection quantity determined by ECU will also decrease accordingly. At this time, the actual intake air volume is relatively large, which leads to excessive dilution of mixed steam, which eventually leads to engine idling jitter and "tempering" when accelerating rapidly.

⑶ The automatic transmission cannot enter the overdrive gear.

If the air flow sensor is short-circuited to the ground, the mixed steam will be too dilute, which will reduce the output power of the engine and cause the automatic transmission to fail to enter the overdrive gear. The air flow sensor should be replaced at this time. Faults of air flow sensors can be divided into two categories. One is that the signal exceeds the specified range, indicating that the air flow sensor is faulty. Modern electronically controlled vehicles have fail-safe function. When the signal of a sensor fails, the electronic control unit (ECU) will replace it with a fixed value, or replace the signal of the faulty sensor with the signal of other sensors. After the air flow sensor fails. ECU replaces it with the signal of throttle position sensor. The other is inaccurate signal (i.e. performance drift). Inaccurate air flow sensor signal may be more harmful than no signal. This is because. Because the signal is not beyond the specified range. The electronic control unit (ECU) will control the fuel injection quantity according to this inaccurate air flow signal, so it will often cause the mixed steam to be too thin or too rich. If there is no air flow signal, ECU will use the signal of throttle position sensor instead, and the idle speed of the engine is relatively stable.

With this function, the performance of the air flow sensor can be judged by pulling out the connector.

① If the fault phenomenon has not changed, it means that the air flow sensor has been damaged. This is because the ECU replaces the air flow sensor with the throttle position sensor signal after confirming that the air flow sensor is invalid. At this time, whether there is an air flow sensor or not is the same, so the fault phenomenon has not changed.

② If the fault phenomenon is alleviated, it means that the performance of the air flow sensor is drifting and the signal is biased. Because the air flow signal is within the effective range, ECU controls the fuel injection quantity according to the distorted signal, which leads to obvious failure. After unplugging the connector of the air flow sensor, ECU thought that the air flow sensor was completely invalid, so it was replaced by the signal of the throttle position sensor, and the working condition of the engine was improved.

(3) If the fault phenomenon has deteriorated. Explain that the air flow sensor is normal. This is because the ECU controls the fuel injection quantity according to the normal air flow sensor signal before unplugging the plug. After unplugging the plug, ECU uses the signal of throttle position sensor to control the fuel injection. Because the control accuracy of the latter is not as high as that of the former, the fault phenomenon is aggravated.

In addition, because the signal of the air flow sensor is the main basis for controlling the air-fuel ratio, the infrared exhaust gas analyzer can be used to measure the exhaust gas composition of the engine at idle speed and 2000 rpm. If it is too different from the standard value, it may be the fault caused by the poor performance of the air flow sensor. (1) When the hot film air flow sensor (G70) fails, the electronic control unit (ECU) does not directly give the fault code of the air flow sensor, but displays it through other fault codes, usually "0056 1" (the adjustment value of mixed steam exceeds the adjustment limit) or "179/kloc".

⑵ Failure of other engine parts may record the fault code of the air flow sensor. In the maintenance practice, the fault code of the air flow sensor is usually recorded under the following circumstances.

① The throttle valve is too dirty, which may record the fault code of the air flow sensor. A Bora 1.8T sedan. In normal driving. Sometimes the ASR indicator light on the dashboard suddenly lights up, and pressing the ASR light switch is invalid. Only by turning off the ignition switch and restarting the engine will the ASR light go out. Connect the scan tool VAS505 1 for testing, and read out the fault message "The signal value of air flow sensor G70 displayed by the engine system is too small". The resistance of each terminal of G70 is not greater than1.5 Ω. Observe the waveform of G70, and it is normal. Replace G70, invalid. The car adopts CAN bus multi-channel information transmission system. ABS/ASR control unit communicates with engine control unit through CAN bus. On the one hand, when the throttle body is dirty, the throttle opening value increases, but the actual intake volume does not increase. The signal of G70 does not match the throttle opening, and the fault message "The signal value of air flow sensor G70 displayed in the engine system is too small" is recorded. On the other hand, the anti-skid control of ASR system is realized by adjusting the output torque of the engine by reducing the engine speed, so although the fault phenomenon is manifested in the braking system, the root cause of the fault is the engine. When there is a deviation between the throttle opening signal and the air flow signal (the throttle opening is large, but the actual air intake measured by G70 is small), ABS/ASR control unit thinks that the engine has reduced the power output and is carrying out anti-skid control, so it lights the ASR indicator light.

② The performance of the throttle position sensor is abnormal, and the fault code of the air flow sensor may be recorded. A Jetta car, detected by a scan tool, reads out the fault code of unreasonable air flow sensor signal. Replace the air flow sensor, but it is ineffective. The so-called "unreasonable" is to compare the signals of relevant sensors. In fact, ECU determines the engine load according to the comparison of engine speed, throttle position signal and air flow signal. Further inspection of the throttle position sensor shows that its maximum learning value and minimum learning value are inconsistent with the specified values, so it is impossible to make basic settings. After replacing the throttle assembly (including the throttle position sensor) and making basic settings, the fault was eliminated.

③ The oxygen sensor is damaged, which may record the fault code of the air flow sensor. When the oxygen sensor of Jetta King car is damaged, it will remember the fault code of the air flow sensor. The principle is that the fuel combustion is incomplete due to "lack of cylinder" and other reasons, which exceeds the adjustment range of λ, resulting in inaccurate oxygen sensor signal, so the engine ECU constantly adjusts back and forth between lean and rich mixed steam. The air flow signal received by ECU contradicts the oxygen sensor signal. However, from the actual effect, like the serious drift of the air flow sensor signal, the oxygen sensor is damaged and cannot be adjusted. The ECU remembers the fault code of the air flow sensor according to the priority principle. (1) The unit and range of signal parameters of air flow sensor depend on the type of air flow sensor. The parameter unit of wing plate, hot wire and hot film air flow sensor is "V", and the range is 0 ~ 5V. The size of this parameter is generally inversely proportional to the air intake, that is, the air intake increases, the output voltage decreases, and "5V" means the air intake is the smallest. "0V" indicates the maximum intake air volume.

The unit of signal parameters of vortex street air flow sensor is "Hz" or "ms", and its range is 0 ~ 0~ 1600Hz or 0 ~ 625 ms, the value at idle speed is 25 ~ 50hz, and the value at 2000r/min is 70 ~ 100hz. If the values under different working conditions differ greatly from the standard values or remain unchanged. This means that the air flow sensor is faulty.

⑵ By analyzing the data flow of the air flow sensor, we can judge whether there is air leakage in the engine intake system. Under normal circumstances, the air flow signal data at idle speed is about 2.5g/s, if it is lower than 2.0g/s, it indicates that there is air leakage in the air intake system; If it is greater than 4.0g/s, it means that the engine has extra load.

An Audi A6 1.8T car with a manual gear. When the engine is running, it will shake every 2 ~ 3 minutes. But the engine starts and accelerates normally. Connect the scan tool VAS5052, input 0 1-08-02, and read the data stream. The airflow data displayed in the fourth area periodically jumps between 0.3 and 3.5g/s ... The inspection found that. The clamp at the lower part of the joint between the air cleaner housing and the air inlet hose is improperly installed, resulting in air leakage. After dealing with the air leakage. Troubleshooting.

(3) During the test, the maintenance personnel will find that the air flow data can still be seen in the data flow after disconnecting the connector of the air flow sensor, and the rapid acceleration response is also sensitive within the normal range. In fact, it is the fault operation mode of the control system, and it is the air flow backup data given by the engine ECU according to the signals such as speed and load. (1) Hot wire and hot film after cleaning.

If the engine has a "backfire" fault, it will often cause serious damage to the air flow sensor. Because the airflow of the engine flows reversely in the intake manifold (that is, "backfire"), which contains carbon particles, it is easy to attach to the sensing element of the air flow sensor, resulting in the following consequences: the signal of the air flow sensor is too large at idle speed, but too small at acceleration and heavy load.

Check whether the hotline has self-cleaning ability: remove the air filter and check the hotline from the air inlet of the air flow sensor. If the engine is turned off for 5s, you can't see the reddish glow of the hotline around 1s, indicating that the self-cleaning ability of the hotline has been lost.

After the hot wire (hot film) is polluted, it can be in the hot and idle state of the engine. Remove the filter screen of the air cleaner, and spray hot wire or hot film directly with carburetor cleaner to remove the carbon deposit attached to it.

⑵ Treatment of damaged hot film air flow sensor.

Many models use the hot-film air flow sensor produced by BOSCH company, and its core part is composed of an integrated circuit (digital/analog conversion circuit) and a Wheatstone bridge, and there is no voltage stabilizing circuit. Therefore, when the power supply voltage is too high or instantaneous high voltage occurs, this hot film air flow sensor is easy to burn out. The reason why the peak voltage of the circuit is too high (greater than 16V) is often that the battery is severely vulcanized, which reduces its capacity and cannot absorb the peak voltage of the generator. Therefore, battery vulcanization is one of the reasons for the damage of the hot film air flow sensor. The solution is to add a 78 12 three-terminal voltage-stabilizing integrated circuit to the front end of the hot film air flow sensor.

(3) Do not measure the way that blocked air enters.

These abnormal ways include: the intake pipe is broken, the vacuum hose is loose, and the intake manifold is not tightly sealed with the cylinder head. If the above situation exists. Some air will directly enter the cylinder without being measured by the air flow sensor, which will eventually lead to the imbalance of mixed steam in the engine.

⑷ The replacement problem after the air flow sensor of Volkswagen train is damaged.

The air flow sensor of Volkswagen is an easy-to-break component. After the air flow sensor is damaged, if the original parts cannot be found for a while, it will face the problem of universal replacement of parts. If the engine is equipped with different types of air flow sensors, the control of fuel injection quantity will be inaccurate. In the open-loop control stage, the fuel consumption of the engine may increase and the temperature of the three-way catalytic converter may be too high; In the closed-loop control stage, the oxygen sensor will constantly correct the concentration of mixed steam, which will make the air-fuel ratio change frequently and eventually lead to the unstable operation of the engine.