An air flow meter is used in some vehicles to gauge the quantity of air that passes into the internal combustion engine. All new electronically powered diesel engines use air flow meter.this is because .it is the only possible means of gauging the air intake. However as In the case of a petrol engine, the ECU calculates how much fuel is needed to pass into the cylinder ports units. But as in the diesel engine, the ECU measures the fuel through the injectors into the engines cylinders during the compression stroke.
The vane (flap) type air flow meters basically measure air volume, while the later “hot wire” and “hot film” air mass meters measure speed of air flow.
The flap type meter has a spring which returns the internal flap to the initial position. If the spring tension is too tight, it can restrict the incoming air and it would then cause the intake air speed to increase when not opened fully. Differential pressure is equally used for air flow measurement purposes. Air flow meters may fail. In situations like that, engine performance will often be decreased significantly, while engine emissions will be greatly increased, and most often usually the malfunction indicator lamb (MIL) will come up. More so some engines do not run at idle with an air flow meter failure.