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The tasks of the air flow meter
On the one hand, the mass airflow sensor measures the quantity of air flowing through, on the other hand it determines the humidity and the temperature of the air drawn in. The values obtained by the measurement are forwarded to the engine control unit. The mass airflow sensor then calculates the amount of fuel required for the optimum composition of the air-fuel mixture and the ignition timing. In diesel engines, the mass airflow sensor also influences the control of the exhaust gas recirculation. The combustion process in the engine is constantly optimized by an exact ratio between air and fuel mass, so that the engine can operate economically and with low emissions. The optimization of the mixture preparation enables compliance with the prescribed exhaust gas standards and the reduction of harmful emissions. The air mass sensor therefore has a very important task in engine management.
The mass airflow sensor is often confused with the older component "airflow sensor", which can only be found in older vehicles. An air flow meter can only determine the volume of the sucked in air. This is not sufficient to adjust the mixture exactly to the respective conditions and circumstances of temperature. Therefore nowadays only the air flow meter is installed.
The designs of the LMM briefly explained
Overall, the LMM can be divided into two designs:
- The hot-wire mass airflow sensor
The older design is the so-called hot-wire air mass meter, which is currently still the most common. This contains a wire which is constantly 100°C above the temperature of the sucked in air. Depending on the driving style more or less air mass is sucked in. The air mass comes into contact with the heating wire and cools it down. Because the temperature of the wire is permanently balanced, the size of the required current can be taken as a measure of the air mass. The engine control unit collects and processes the information and regulates the corresponding fuel quantity. The air mass is measured about 1000 times per second. After the engine is switched off, the hot wire is heated briefly to a very high temperature of about 1000 °C. The purpose of this is to burn off impurities and to keep the LMM functioning properly. - The hot-film mass airflow sensor
The newer design is the so-called hot film air mass meter, which works with a sensor plate instead of a heating wire. Also with this type of air mass sensor, the air mass is determined by the temperature difference measured by the sensor and the intake air. The result is then transmitted to the engine control unit and the required fuel quantity is calculated. An essential advantage over the heating wire air mass sensor is that no exhaust gas particles are deposited. This means that it is no longer necessary to burn off the impurities. Modern air mass sensors measure humidity and pressure signals in addition to temperature.
Possible symptoms of a defective air mass sensor
Air flow meters also wear out over time. The following symptoms may indicate this:
- loss of motor power, fluctuations at low speed
- unstable idling and starting problems
- poor exhaust gas values including black smoke development
- The engine indicator light comes on.
If these signs are due to a defective LMM, it transmits an incorrect voltage signal to the control unit. This results in too little or too much fuel being injected. The causes can be deposits such as dried oil, water or fuel residues as well as dust or dirt. These usually reach the air mass sensor due to a defective or dirty air filter. Therefore it is also important not to forget the engine air filter during the inspection. But other components in the intake system, ignition system or turbocharger can also cause the above-mentioned complaints. Troubleshooting should be carried out professionally, as a lot of components can cause similar symptoms. In some cases, cleaning the mass airflow sensor is sufficient so that the sensor measures the correct values again. However, care should be taken here as the sensor parts are very sensitive to touch. Driving on with a defective LMM leads to a much higher fuel consumption and, if present, to switching to the emergency program. At the same time the engine power is throttled down. It is therefore advisable to determine the exact cause of the problem as quickly as possible.
Clean or replace the air mass sensor?
If a LMM no longer functions as it should, some people try to clean the air flow meter first. For this purpose, newer vehicles require special tools and generally a special cleaner. As this is a very sensitive technique, experience and tact is required. It is not always crowned with success to clean an air flow meter, and then an exchange becomes necessary after all.
If the old LMM has to be replaced, the amount of work is not very large. Usually only the engine cover and the air intake pipe have to be removed. Caution is nevertheless necessary, because the air mass sensor is very sensitive. With a newer LMM you may need special tools. Depending on the vehicle manufacturer, it may be necessary to re-teach the control unit after installation of the new air mass sensor, so that the engine management system is informed about the new component. In some vehicles, the mass airflow sensor is integrated in the engine control unit, which makes repair much more complex and also more expensive.
We recommend high-quality replacement parts, especially for a sensitive component such as the air flow meter. Bosch, for example, is the original equipment manufacturer for many BMW and Mercedes-Benz vehicles. But you can also find Pierburg, Hella, Herth+Buss, Magneti Marelli and many other well-known manufacturers at reasonable prices. If you are unsure which LMM is suitable for your vehicle, please contact our customer service with the 17-digit chassis number of your vehicle.
