Q: What Does the Mass Airflow Sensor Do?

Older systems use a movable vane in the intake stream called a Vane Air Flow (VAF) sensor. The VAF sensor used in Bosch L-Jetronic, Ford, and most Japanese electronic port fuel-injection systems is a movable vane connected to a laser-calibrated potentiometer. The vane is mounted on a pivot pin and is deflected by the intake airflow proportionate to air velocity. As the vane moves, it also moves the potentiometer. This causes a change in the signal voltage supplied to the computer.

All newer fuel-injection systems use a Mass Air Flow (MAF) sensor to calculate the amount of air volume delivered to the engine. There are several types of mass air flow sensors.

• Karman Vortex Sensors: This type of sensor sends a sound wave through the turbulence created by incoming air passing through the sensor. Air mass is calculated based on the time required for the sound waves to cross the turbulent air passages. There are two basic designs of Karman vortex airflow sensors.

• Ultrasonic type of sensor uses ultrasonic waves to detect the vortexes that are produced and produces a digital signal where frequency is proportional to the amount of air passing through the sensor.

• Pressure type uses a pressure sensor to detect the vortexes. As the airflow through the sensor increases, so do the number of pressure variations. The electronics in the sensor convert these pressure variations to a digital DC signal, whose frequency is in proportion to the airflow through the sensor.

• Hot Film Sensor: The hot film sensor uses a temperature sensing resistor to measure the temperature of the incoming air. Through the electronics within the sensor, a conductive film is kept at a temperature of 70°C above the temperature of the incoming air.

• Hot Wire Sensor: The hot wire sensor is similar to the hot film type but uses a hot wire to sense the mass airflow instead of the hot film. Like the hot film sensor, the hot wire sensor uses a temperature sensing resistor to measure the temperature of the air entering the sensor. These airflow sensors are used mostly to determine the amount of fuel needed. The greater the mass of incoming air, the longer the fuel injectors are pulsed on. Of these sensors, the hot film type and hot wire type sensors are the most common.

Both designs operate in essentially the same way. A resistor wire or screen installed in the path of the intake airflow is heated to a constant temperature by electric current provided by the computer. Air flowing past the screen or wire cools it. The degree of cooling varies with air velocity, temperature, density, and humidity.

These factors combine to indicate the mass of the air entering the engine. As the screen or wire cools, more current is required to maintain the specified temperature. As the screen or wire heats up, less current is required. The operating principle can be summarized as follows: * More intake air volume = cooler sensor, more current. * Less intake air volume = warmer sensor, less current.

The computer constantly monitors the change in current and translates it into a voltage signal that is used to determine injector pulse width.

MAF Sensor Contamination: Dirt, oil, silicon, or even spider webs can coat the sensing wire. Because it tends to insulate the sensing wire at low airflow rates, a contaminated sensor often overestimates the amount of air entering the engine at idle and causes the fuel system to go rich. At higher engine speeds near wide-open-throttle, the contamination can cause the sensor to underestimate the amount of air entering the engine. As a result, the fuel system will go lean, causing spark knock and lack of power concerns.

Some MAF-related diagnostic trouble codes include: * P0100 - Mass or volume airflow circuit problems. * P0101 - Mass airflow circuit range problems. * P0102 - Mass airflow circuit low output. * P0103 - Mass airflow circuit high output.

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle if you see symptoms of a bad or failing mass airflow sensor. Get a quote and book an appointment online to replace the mass airflow sensor or speak to a service advisor at 1-888-324-3543.