The amount of air entering the engine must be measured, so that the amount of fuel injected into it forms a mixture to suit the engine operating conditions at that time.
The volume of air entering the engine varies according to the position of the accelerator pedal, and the engine speed, and load.
Changes in volume are reflected in changes in manifold pressure, between the throttle plate and the intake ports. When the pistons draw air away from the manifold more quickly than air can pass the throttle plate into it, the pressure in the manifold falls.
At idle speeds, this pressure is low. A high depression, or a vacuum, exists. As the throttle plate opens, the pressure approaches that of the atmosphere. The depression or vacuum tends to fall in value.
Devices measuring air entering the engine send a varying electrical signal to the ECU. The signal can be a measure of the volume of air, or of the manifold pressure. In both cases, this information is combined with the engine speed signal, to provide the basic setting for the injector pulse width.
An air-flow meter measures the volume of filtered air entering the engine. It can be mounted on the air filter casing, or close by. All air entering the intake flows through the meter, then through a sealed, flexible hose or duct to the throttle body, then enters the engine.
Vane-type air-flow meters have a spring-loaded vane, or flap, which deflects according to how much air enters. The electrical signal to the ECU varies with this deflection, as contacts attached to the vane move across a potentiometer.
The intake system must be air-tight, with no leaks downstream of the meter, otherwise inaccurate signals will be sent to the ECU.
