A mass air flow sensor is an important component in engines which utilize fuel injection instead of carburettors. Mass airflow sensors, or MAF sensors for short, measure the density of the air passing through an engine’s intake and report to the ECU (engine control unit) computer, allowing it to deliver the right amount of fuel through the injectors and into the combustion chambers. Air density changes based on temperature, altitude and the use of forced induction, so it is important for fuel injected engines to able to read air density and adjust the amount of fuel delivered through the injectors accordingly. Fuel economy, engine power and general correct engine operation depend on maintaining the correct air/fuel ratio. A faulty MAF sensor can rob an engine of power and fuel economy as well as causing it to idle incorrectly and stall.
Types of Mass Air Flow Sensor
Moving Vane Meter
Older fuel injected engines use what’s known as a moving vane MAF sensor. In this configuration, the sensor contains a small vane/door/flap which is pushed open by the air passing through the engine’s intake. By measuring how much the vane is opened by the intake air the sensor is able to work out the flow of air entering the engine through the intake. It then relays this information to the ECU which adjusts the fuel delivered to the injectors accordingly.
Drawbacks of Moving Vane MAF Sensors
- Design restricts airflow which limits the amount of power an engine can produce.
- Moving mechanical and electrical components are susceptible to fatigue and can fail more often than other MAF sensor designs.
- It can be difficult to find a good spot to mount the sensor in a tightly-packaged engine bay.
Hot Wire MAF Sensor
In this configuration a platinum wire is suspended in the path of the intake airflow through which a current is passed at a constant voltage. As the wire’s temperature increases its electrical resistance also rises. The air moving through the engine’s intake passes over the wire and cools it which lowers its electrical resistance. The sensor then converts this difference in resistance to a signal that the ECU can understand, allowing it to alter the amount of fuel sent to the injectors and keeping the air/fuel ratio as close to optimal as possible. Hot wire MAF sensors are more accurate than moving vane sensors and are found on more modern fuel-injected engines.
Benefits of Hot Wire MAF Sensors
- No airflow restriction because the sensor doesn’t rely on a small gate being pushed open, just air moving over and past a wire.
- Responds more quickly and accurately to changing air density/flow than vane style.
- Smaller package, doesn’t mind where it’s mounted as much.
- No moving parts mean that it’s more durable.
- Dirt and oil can attach to the wire and damage its accuracy. Special MAF sensor and electronics cleaners should be used if the sensor wire becomes dirty as carburetor and brake cleaners can be too strong and further damage the sensor wire.
- Thin and delicate platinum wire can be damaged if handled incorrectly.
Cold Wire MAF Sensors
Works like a hot wire system, but adds a second cold wire to provide a baseline reading with which the hot wire’s reading can be compared. This allows the sensor to provide a more accurate reading of the density of air passing through the engine’s intake.
Symptoms of a Failing or Damaged MAF Sensor
- Running rich at idle (too much fuel in the fuel/air mixture.
- Running lean under load (too little fuel in the mixture when the driver’s foot is on the gas pedal)
- Decrease in fuel efficiency
- Rough, uneven idle where the engine refuses to stay at an even RPM.