Air Flow Metre AKA Air Mass Sensor - What are the symptoms or signs of it failing ?

Technical

Nov 27th, 2018

Air Flow Metre AKA Air Mass Sensor - What are the symptoms or signs of it failing ?

The mass airflow sensor, commonly referred to as the airflow metre, is undoubtedly one of the least known parts of a car and one of the components that get little attention from car owners. It is part of the electronic fuel injection in internal combustion engines that do not use carburettors. 

Sounds mind-boggling? Do not worry, Carparts.com.au experts are here to shed some lights on the air mass sensor and precisely what it does.

What is the function of a mass airflow sensor?

As you probably already know, for internal combustion to take place in the engine of your car, oxygen is needed. More specifically, for optimal combustion to take place, a precise balance of air and fuel is required. As the driver accelerates or decelerates, the throttle plate/butterfly in the throttle body opens or closes, allowing in more air or less air respectively in response to the driver's action. With this increase and decrease in the air intake, the amount of fuel injected needs also to vary accordingly to maintain the specific balance of air and fuel. To facilitate this variance, this is where the AFM (air flow metre) comes in. 

As the throttle body let in the air in response to the drivers push on the accelerator, the AFM measures the mass of air intake and sends this information to the vehicle’s Electronic Control Unit (ECU). The ECU interprets the data on air mass flow rate and in return, communicates the correct mass of fuel to be injected to achieve the required air-fuel ratio for optimum fuel combustion.

There are several types of airflow sensors, and their primary differentiation is the technology they apply to measure the mass of airflow. Here are the different types:

  • Hot-wire mass airflow (MAF) sensor - the most common in modern vehicles and uses a small electrically-heated wire. The electric conduction resistance of the hot wire increases with temperature, i.e. the higher the temperature, the higher the resistance and the less the electric current. The hot wire connects to an electronic chip installed inside the AFM. The chip deciphers the electric current into digital signals and communicates with the engine computer (PCM or ECU) in terms of air-mass flow rate. When air passes through the sensor, it cools the hot wire. The higher the air-mass flow rate, the colder the wire gets, and the more the current passes through the cable to the chip. The chip in return sends the signal to the PCM for the right amount of fuel to be injected.
  • Moving vane meter - this volume airflow sensor is an older generation tech in fuel-injected engines and has a flap/butterfly/vane that is spring-loaded and is opened by the air inflow. The flap opens in proportion to the air mass flow rate. The vane type of a sensor also has a potentiometer, which connects to the moving flap. Electric current passes through to the outer terminal of the potentiometer. As air passes through, the flap is forced to open, and the angle of the opened flap determines the output voltage at the output terminal of the potentiometer. The voltage output is in return sent as digital signals to the ECU to determine the amount of fuel to be injected. Other types of vane meter, such as those used in K-Jetronic have the movement of the flap directly controlling the amount of oxygen.
  • Other sensors include membrane sensors in emerging technologies, such as cold-wire sensor, Karman Vortex sensor, and Laminar flow sensor.

Symptoms of Bad or Damaged Mass Air Flow Sensor

Unfortunately, the mass airflow sensors are not damage-proof, and at times they malfunction. When they malfunction, they send the wrong information to the ECU, leading to the injection of an incorrect amount of fuel. You will know when this happens by watching out for the following signs and symptoms:

  • Change in fuel economy - a common symptom is a change in fuel consumption. A malfunctioning sensor may also send the long signals resulting in the injection of more or less fuel than required for the given air mass flow rate. Injecting less fuel will result in your vehicle consuming less fuel but having a significantly lower power. Injection of more fuel than needed, on the other hand, will lead to consuming more fuel than usual without significant change in power (referred to as "running rich").
  • Engine runs too lean - injection of less fuel than needed due to malfunctioning mass airflow sensor results to a low fuel-to-air ratio (referred to as "running lean"). Running lean is when there is more air than fuel in the combustion chamber, resulting in less power. In effect, the driver will feel the need to push the vehicle harder in an attempt to get the usual vehicle power. Pressing the vehicle's accelerator harder will only end up in the car consuming more fuel and getting fewer kilometres per litre of fuel than usual. 
  • Uneven idling of the engine - it is not uncommon when a vehicle has issues with airflow metre to start idling roughly, especially in the early stages of the problem. Any inconsistency in sending of information or slight error in the information submitted to the PCM causes the injection of an incorrect amount of fuel resulting in uneven idling. By keenly observing the tachometer on your vehicle’s dashboard, you will see the RPM speed variations.
  • Weak and inconsistent acceleration - when your mass airflow sensor has issues, and you push on the accelerator, the sensor may delay in sending the signals or send an inconsistent signal or the wrong signal. These signal errors may result in delayed or irregular acceleration. At times, the vehicle may exhibit some jerking movement as you continuously press on the accelerator. 
  • Vehicle stalls down eventually - if you don't address the problem promptly, the airflow metre may finally stop communicating with the ECU. As a result, the ECU also stops sending signals to the fuel injector and no fuel is injected, leading to the vehicle stalling down.

Handy Hint: Unlike in the earlier days, diagnosing a faulty air flow sensor is now a quick process. An engine expert with the right tools can easily detect a defective air mass sensor and repair or replace it within an hour or so. This advancement beats the old days when pinpointing an engine problem used to be a trial-and-error thing and different components of the engine had to be tested to find out the exact problem with the car.