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MAF (Mass Air Flow)
Principles
Function of the mass air flow sensor
The mass air flow (MAF) sensor is part of the Engine Management System. It is sometimes also known as an 'Air Flow', 'Air Mass' meter or sensor.
It determines the amount of air flowing into the intake system of diesel and petrol internal combustion engines.
By means of the air mass information the engine control unit (ECU) is able to balance and deliver the correct fuel quantity to the engine. In diesel engines, the MAF sensor serves mainly to control the exhaust gas recirculation (EGR).
Development of the mass air flow sensor
In the past, mechanical meters were used to measure the intake air volume. In these VAF (volume air flow) meters, the air flows against a spring-loaded air vane (flap) attached to a variable resistor (potentiometer). Depending on the electrical resistance, the engine control unit (ECU) determines the intake air quantity.
Nowadays the majority of MAF sensors come in two variations: hot wire technology and hot film technology.
Unlike a VAF meter, these MAF sensors have no moving parts and use electrical current to measure air mass.
Installation position of MAF sensor
The Mass AirFlow sensor is located between air filter and intake manifold.
Construction
Construction 1
There are different variations of airflow in MAF sensors.
In this version, only a part of the air flows past the measuring element in the measuring channel.
Construction 2
Air flow through the sensor
The screen at the front of the MAF sensor is designed to align the turbulant air so that the sensor reads the airflow more precisely.
Top: without screen
Bottom: with screen
Construction 4
Measuring channel
Cover
Sensor element
Partial air flow (Bypass)
Housing
Electrical circuit
Operating Principles
Operating Principle Mass Air Flow Sensor
Move the slider to increase engine speed
Operating Principle Hot-Wire Mass Air Flow Sensor
The Hot-Wire Air Mass Sensor has a measuring element that is heated electrically to a certain temperature (75° - 100°C) above the intake air.
When the air flow passes the measuring element, it cools the element and increases the current needed to keep the element hot.
The cooling effect varies directly with the temperature, density and humidity of the incoming air.
The amount of current needed to keep the element hot is proportional to the air mass entering the engine.
Operating Principle Hot-Film Mass Air Flow Sensor (Variant I)
The functional principle of the Hot-Film MAF sensor, is similar to the hot-wire technology.
A thin film heating element is heated to a certain temperature with respect to the intake air.
At zero flow there is heat dissipation due to the thermal conductivity of the air.
With applied flow the heat dissipation increases steadily with the air flow rate.
By means of the required heating current, the electronics are able to calculate the air mass.
This system has the following advantages compared to the hot-wire MAF sensor:
- Increased measuring response due to smaller sensor element
- Less sensitive to particles in the intake air
Operating Principle Mass Flow Sensor
A variant of the Hot-Film Air Mass Meter has 2 measuring elements with a heated ceramic element lying in between.
The thin film heating element is maintained at a constant temperature above intake air, regardless of air flow.
When the engine is not in operation (without air flow), the heat is distributed evenly to both sensors. Both sensors measure the same air temperature.
When the engine is running the intake air flows from T1 in the direction of T2 over the sensor element. The air cools sensor T1.
The air heats above the heating element, so that the sensor T2 cools down less than T1.
By means on the temperature difference between sensor T1 and T2, the electronics is able to calculate the air mass.
Operating Principle Hot-Film Mass Air Flow Sensor With Backflow Detection
In order to minimize measurement errors caused by the pulsating air in the intake manifold, some MAF sensors have backflow detection.
Operating Principle Hot-Film Air Mass Meter With Backflow Detection
In order to minimize measurement errors caused by the pulsating air in the intake manifold, mass airflow sensors with backflow detection will be used.
If air flows back from the engine side, instead the temperature sensor T2 is cooled and the temperature sensor T1 is heated. This back flow causes an opposite temperature difference at the sensors (T2 and T1), which is taken into account in the precise air mass calculation by the electronics.
Diagnosis
Air Mass flow sensor I
Diagnosis of a Mass Air Flow sensor
While driving:
- Gear 3 or 4, full throttle (max boost pressure)
- Look in parameter list in scan tool
- Compare calculated and measured value, they should correspond
Air Mass flow sensor II
Diagnosis of a Mass Air Flow sensor
Engine off:
- Ignition on
- Remove air hoses from MAF sensor
- Leave the electrical connector plugged into sensor
- Blow air into the MAF sensor e.g. with a compressed air gun (be careful, not too close!)
- Voltage output should rise, then remain constant between 4.2 - 4.8 Volt.
Air Mass flow sensor III
Diagnosis of a Mass Air Flow sensor
If engine (petrol) does not start at all:
- Disconnect electrical connector of MAF sensor --> ECU switches into the emergency operating mode
- If air mass meter is defective/polluted: engine starts / runs better --> Note that cleaning of a dirty sensor element is rarely successful and in order to restore full engine performance a new unit should be installed
Air Mass flow sensor IV
Diagnosis of a Mass Air Flow sensor
Just because the engine control unit is reporting a MAF fault code, it does not necessarily mean that the MAF sensor is at fault. Unmetered air from a split intake hose, crank case ventilation breather pipe or even a sticking EGR (Exhaust Gas Recirculation) valve can all trigger a MAF fault code.
Air Mass flow sensor V
How the ECU determines the correct function of a MAF sensor:
The ECU compares the “theoretically” (= calculated) aspired amount of air with the measured amount.
If a mismatch is detected, a Fault code (also known as a Error Code/Diagnostic Trouble Code/DTC) will be stored. This is not necessarily caused by a fault with the MAF sensor!
See the animation for details.
Air Mass flow sensor VI
Diagnosis of a Mass Air Flow sensor
MAF and lambda information is used by the ECU to check and adjust fuelling. Therefore contaminated, faulty or deteriorated/lazy lambda sensors can also trigger MAF fault codes.
Air Mass flow sensor VII
Diagnosis of a Mass Air Flow sensor
A common occurrence after the replacement of a faulty MAF sensor is the vehicle not idling correctly, a flat spot on acceleration or a re-appearing fault code, leading to the sensor being incorrectly blamed again.
It is not uncommon for an engine to run a little rough and not idle properly initially after replacement.
A road test is often needed for other actuators and sensors on the engine to re-calibrate and adjust.
In some cases the adaptation values or other parameters may need to be reset using the relevant diagnostic equipment.
Keeping all these factors in mind when working with MAF sensors, helps to fix the fault first time.
Note that any diagnosis described in this article covers basic checks which may not be applicable to all MAF sensor operating princilples.
Air Mass flow sensor VIII
Diagnosis of a Mass Air Flow sensor
There are sometimes reasons for the problem other than a faulty MAF sensor, which have the same or similar symptoms and sometimes produce the same fault code.
The most common problems with similar symptoms are:
- Defective/leaking vacuum or intake hoses
- Defective/damaged cable and plug connections of the air mass meter
- Clogged and/or soiled air filters or intake manifold
- Defective throttle
- Polluted fuel filter
Air Mass flow sensor IX
Diagnosis of a Mass Air Flow sensor
When replacing a MAF sensor it is good practice to replace the air filter at the same time.
