A mass air flow sensor, often called a MAF sensor, measures how much air enters an engine through the intake system. This matters because the engine control module must mix the correct amount of fuel with that air for efficient combustion. Too much or too little fuel can reduce power, waste fuel, and increase emissions.
In most cars, the MAF sensor sits in the air intake tube between the air filter and the throttle body.
Key Facts
- The MAF sensor measures the mass of incoming air, not just its volume.
- A common location is between the air filter and the throttle body in the intake tube.
- Air fuel ratio for gasoline engines is often near 14.7:1 by mass during steady cruising.
- Fuel mass needed = air mass / air fuel ratio.
- Hot wire MAF sensors estimate airflow by measuring how much electrical current is needed to keep a wire hot.
- Dirty air filters, air leaks, or a contaminated sensor can cause incorrect MAF readings.
Vocabulary
- Mass Air Flow Sensor
- A sensor that measures the mass of air entering an engine so the computer can calculate fuel delivery.
- Engine Control Module
- The vehicle computer that uses sensor data to control fuel injection, ignition timing, and other engine functions.
- Throttle Body
- A valve assembly that controls how much air flows into the engine when the driver presses the accelerator.
- Air Fuel Ratio
- The comparison of the mass of air to the mass of fuel in the mixture burned by the engine.
- Hot Wire Sensor
- A type of MAF sensor that uses a heated wire and airflow cooling to determine how much air is entering the engine.
Common Mistakes to Avoid
- Confusing air mass with air volume. This is wrong because hot, cold, high pressure, and low pressure air can have different mass even if the volume is the same.
- Assuming the MAF sensor controls airflow directly. This is wrong because the throttle body controls airflow, while the MAF sensor measures it.
- Ignoring intake air leaks after the MAF sensor. This is wrong because unmeasured air can enter the engine and make the fuel mixture too lean.
- Cleaning a MAF sensor with the wrong spray or touching the sensing wire. This is wrong because the delicate sensor can be damaged or contaminated, causing inaccurate readings.
Practice Questions
- 1 A gasoline engine receives 147 grams of air during a short operating interval. Using an air fuel ratio of 14.7:1, how many grams of fuel should be injected?
- 2 A MAF sensor reports an airflow of 60 grams per second. If the engine runs for 5 seconds at this airflow, what total mass of air enters the engine?
- 3 Explain why a crack in the intake tube between the MAF sensor and the throttle body can cause rough running even if the MAF sensor itself is working correctly.