⑴Causes poor engine acceleration
A Passat GLi sedan, with a mileage of 45,000 km, accelerates the engine to 4200r/min, and then steps on the accelerator pedal, the engine speed drops instead. Use VAG1551 fault diagnosis instrument to detect, no fault code storage. Reading the data flow at 4200r/min, it is found that the air flow data can only reach 1.1-1.3g/s, and it cannot change with the opening and closing of the throttle. After replacing the air flow sensor, the fault was eliminated. The reason. The output signal deviation of the air flow sensor is not enough for the electronic control unit (ECU) to record the fault code, but because the air flow signal cannot accurately reflect the actual intake air volume, the ECU controls the fuel injection volume accordingly, so the engine speed Decline instead of rising.
⑵Causing "backfire" in the intake pipe
A Jetta King sedan. The engine idling jitter occurs, and the intake pipe "backfires" during rapid acceleration. Check the air intake system, no leaks were found. Replacing the fuel filter and cleaning 4 fuel injectors is invalid. Check the fuel pressure, it is normal when idling and accelerating. The connector of the air flow sensor was removed and tested, and the malfunction phenomenon was greatly improved. Measure the resistance value of each terminal of the air flow sensor, and it is normal. Finally, it was found that there was dust on the hot film resistor of the air flow sensor. After cleaning the dust with vaporizer cleaning agent, the fault was eliminated.
For electronic fuel injection engines using hot film air flow sensors, a constant voltage is applied to both ends of the hot film (resistor) to make the resistor heat, and its temperature is controlled by the circuit. ECU judges the amount of intake air according to the current flowing through the thermal film resistor, and determines the amount of fuel injection to meet the needs of different engine operating conditions. If dust accumulates on the hot film to form a thermal insulation layer, when the air intake volume increases, its temperature change slows down, and the required current decreases, and the fuel injection volume determined by the ECU based on this will decrease. At this time, the actual intake air volume is relatively large, which causes the mixed steam to be too lean, and eventually causes engine idling jitter, and "backfire" during rapid acceleration.
⑶The automatic transmission cannot be raised to overdrive
If the air flow sensor is short-circuited to the ground, it will cause the mixed steam to be too lean and the output power of the engine will drop, which will cause the automatic transmission to fail to go into overdrive. The air flow sensor should be replaced at this time.