Airflow Meter Operation
The airflow meter is located in line with the intake air duct, between the air cleaner and the intake manifold. All engine air is drawn through the airflow meter, which contains a tunnel with a measuring flap and dampening flap (offset 90° on the same casting). The measuring flap swings in the air stream against the pressure of a calibrated spring. A potentiometer connected to the flap supplies the ECU with a voltage signal. This signal is directly proportional to the degree of flap opening (air flow).
A temperature sensor is fitted in the airflow meter to measure the temperature of incoming air. Resistance value of the sensor varies with air temperature. A voltage signal from the sensor combines with airflow meter output voltage, resulting in a voltage signal to the ECU which indicates air density as well as volume. This information is used to determine engine fuel requirements under various conditions.
At idle, the air flap is almost completely closed. Idle air requirements are met by an airflow bypass built into the meter. Idle air flow rate can be adjusted with the airflow bypass adjustment screw, although adjustment should not be necessary under normal operating conditions. The screw is sealed with a tamper resistant cap which should only be removed, and bypass adjusted, during a major engine overhaul, when a new airflow meter is installed, or if CO readings are excessively high.