Quantity Flow Meter

An air flow meter, also known as an air consumption meter, is a device that measures how much air is flowing through a tube. It does not measure the volume of the air passing through the tube; it measures the speed of the air flowing through the device in a defined time segment. Thus air flow meters are simply an application of mass flow meters for a special medium. Typically, mass air flow measurements are expressed in the units of kg/h (kilograms per hour).
Air flow meters are also referred to as mass air flow meters, abbreviated MAF. An air flow meter is used in some cars to measure the quantity of air going to the engine. All modern electronically controlled Diesel engines use air flow meter as it is the only possible means of determining the air intake for them. In the case of a gasoline engine the electronic control unit then calculates how much fuel is needed to inject into the cylinder ports.
In the diesel engine the ecu meters the fuel through the injectors into the engines cylinders during the compression stroke. The vane (flap) type air flow meters (Bosch L-Jetronic and early Motronic EFI systems) actually measure air volume, whereas the later "hot wire" and "hot film" air flow meters measure speed of air flow.
The flap type meter includes a spring which returns the internal flap to the initial position. Sometimes if the spring is tensioned too tight, it can cause restrict the incoming air and it would cause the intake air speed to increase when not opened fully.