Air Mass Sensor: Expensive Weakness in VW/Audi TDI (Articles)

Mass Air Flow Sensor (MAF): An expensive weak point in TDI engines.

What is the purpose of the mass airflow sensor (MAF sensor, sender G70)?

The LMM (Lambda Measuring Module) provides an important signal for calculating the fuel injection quantity, which is based on the mass of air entering the engine. Air mass measurement, on the other hand, is only used in some older diesel models. The air flow meter used for air mass measurement functions fundamentally differently from an air mass sensor and is not the subject of this article.

The more detailed relationships are shown in https://community.dieselschrauber.org/en/viewtopic.php?t=8670.

For the operation of the TDI, a statement about the fresh air mass per cylinder filling is required (unit of measurement: mg/H).
The mass airflow sensor (MAF) only measures the air flow towards the engine, which can be just as high at low RPM and high boost pressure as it is at higher RPM and lower boost pressure.
The engine control unit (ECU) therefore divides the mass airflow signal (simplified, this means it divides the total air entering the engine) by the engine speed and calculates the amount of fresh air available per intake stroke in each cylinder.


The mass airflow sensor (MAF) voltage also serves as a feedback signal for the exhaust gas recirculation (EGR) system: When the EGR valve is open, a portion of the normally drawn fresh air is replaced by exhaust gases, which causes the MAF value to decrease accordingly. The EGR system is controlled in such a way that, depending on the engine's operating conditions (boost pressure, speed, and fuel injection volume), the target value of the MAF signal, as defined by the EGR map, is achieved.


A few technical facts about the LMM.

The mass airflow sensor (MAF) is located between the air filter and the turbocharger, and it measures the amount of air being drawn into the engine.

The operating principle of the hot-wire anemometer (HWA) has been further developed from earlier hot-wire mass flow meters.
TDI mass airflow sensors essentially consist of thin, temperature-sensitive sensor membranes on a small electrical "heater" in the form of a plate that is exposed to the intake air stream.
The airflow cools the sensor membrane, which in turn changes the electrical resistance value of the membrane.
This change in resistance forms the basis for generating the actual LMM signal voltage, which is achieved through integrated (analog) amplification and processing electronics within the LMM.

The differences between the Pierburg mass airflow sensors (MAF) used until approximately 1998 and the subsequent Bosch MAF sensors are primarily interesting due to the electrical connections and voltages at the signal pin.

The newer Bosch development not only detects the mass flow but also the direction of the airflow, which is advantageous for accurately measuring the actual air volume when dealing with highly pulsating air columns (due to the opening and closing rhythm of the intake valves) at low flow rates.
This requires a higher initial voltage without airflow to ensure that backflows can be detected by reporting a correspondingly lower voltage.

Pinout of the mass airflow sensor

Pierburg: one-piece mass airflow sensor, 6-pin connector with 5 used pins
1 = Supply + 5 Volts.
2 = Mass (sensor string).
3 = Power supply + 12 volts.
5 = Mass (battery/body).
6 = LMM signal.

Bosch HFM 5: two-piece mass airflow sensor (housing and sensor insert), 5-pin connector with 4 used pins, analog output signal
2 = Power supply + 12 volts.
3 = Mass (sensor string).
4 = Supply + 5 Volts.
5 = LMM signal.

Bosch HFM 6: two-piece mass airflow sensor (housing and sensor insert), 5- or 4-pin connector with 4 pins, analog or digital output signal
1 = Power supply + 12 volts.
2 = Mass (sensor string).
3 = Supply + 5 Volts.
4 = LMM signal.

respectively / or rather / that is to say

1 = Power supply + 12 volts.
2 = Mass (sensor string).
3 = free
4 = Supply + 5 Volts.
5 = LMM signal.

The pin numbering is clearly visible on the inside of the MAF sensor; for Bosch MAF sensors, pin 1 is located on the rounded side of the connector.

To directly monitor the LMM signal using a multimeter (with as high an impedance as possible), the signal cable must be tapped into.
The analog signal voltage relative to ground for a stationary engine is approximately 0.3 volts for a Pierburg mass airflow sensor (MAF), and about 1 volt for a Bosch MAF.
At idle, the voltages are approximately 1 volt higher without the EGR system active.
The voltage increases with the engine speed and turbocharger pressure, reaching a maximum of approximately 4.5 volts for both types.
Significant discrepancies in voltage readings (even with a seemingly intact engine and vehicle-side electrical/electronic systems) can indicate contamination or defects in the mass airflow sensor (MAF).

For sensors with a digital output signal, testing can only be performed using an oscilloscope, or preferably a diagnostic system. It is a square wave signal where the frequency increases proportionally to the measured air mass.

If a diagnostic system such as VCDS (VAG-COM) is available, you can directly read the corresponding measurement blocks from the engine control unit and compare whether the actual value of the measured air mass is at least equal to the target value.

The output signal from all mass airflow sensors (MAF) is already "cleaned" of temperature and pressure influences by internal compensation circuits. Therefore, it reports to the engine control unit (ECU) how much air mass flows through the MAF sensor per unit of time in the direction of the engine.
To enable the injection quantity and EGR rate to be adjusted as quickly as possible during changes in engine speed and load, a correspondingly short response time of the mass airflow sensor (MAF) signal is required for changes in mass flow. This means that the sensor membranes must be able to react to the "new" temperature as quickly as possible.
This requires a minimal intrinsic heat storage capacity, which is achieved through tiny sensor plates and layer thicknesses of just a few thousandths of a millimeter.
The minimal layer thickness results in the high sensitivity of the LMM sensor plates to any contact with solid objects, which usually leads to damage to the membranes and renders the LMM unusable.


"When the mass airflow sensor (MAF) is defective, it usually reports a lower than actual air mass flow rate. For the EGR control system, this appears as if the EGR valve is already partially open. As a result, the EGR rate is reduced by the amount that the MAF sensor underreports."
Since there appears to be less air mass available for combustion in the upper load range, the injection quantity is reduced accordingly. This results in a loss of engine power, which frequently leads TDIs to workshops, where they are often fitted with a new mass airflow sensor (MAF) at considerable expense – provided the cause is correctly identified.

Attention: Defective mass airflow sensors with excessively low readings generally do not cause an entry in the error memory, even if the driver perceives clear faults (e.g., significant loss of power, possibly pulsating acceleration...).
A fault code will only be logged in cases of severe electrical defects, such as a short circuit in the mass airflow sensor signal to ground or to the power supply voltage, etc.

Depending on the size of the mass airflow sensor (MAF) error, fuel consumption will also increase: The boost pressure continues to be regulated based on the (theoretically) verschandeln combustion of the amount of fuel injected as requested by the accelerator pedal, while the actual injection amount may be significantly lower due to the faulty MAF sensor.
That is to say, the boost pressure is being set too high (measured by the actual fuel injection amount), which wastes unnecessary energy -> and increases fuel consumption.


Common causes of Mass Air Flow (MAF) sensor failures.

"Severe" vibrations (caused by design flaws in the vehicle or incorrect installation of the mass airflow sensor) can stress the sensor elements to the point of breakage, causing the mass airflow sensor to fail suddenly and completely.

Any deposits on the membrane will reduce the mass flow rate measurement because they impede the heat transfer from the sensor to the flowing air. Deposits typically form...
-> due to oil vapor from the crankcase ventilation system, which, after the engine is turned off, spreads "backwards" through the intake manifold towards the mass airflow sensor (some 6-cylinder engines are particularly problematic in this regard).
-> due to fine foreign particles (dust) and other vapors that pass through the air filter.
-> due to water that, during driving in the rain (especially from spray kicked up by the vehicle in front), can eventually penetrate the air filter and also permanently damage the mass airflow sensor - particularly in winter, when aggressive road salt dissolved in the spray is present.

In addition, the mass flow meter itself ages over time, which becomes noticeable during its operational lifespan through decreasing readings for the same mass flow rate.
The result is a (usually gradual) loss of engine power.
Often, after a mileage of around 100,000 km, the catalytic converters are so dirty or aged that the engine noticeably loses power.


Given that, based on general experience, the mass airflow sensor (MAF) is often the sole cause of performance loss, this problem is frequently resolved by simply replacing the MAF sensor.
Unfortunately, this rule of thumb often leads workshops to do exactly that – perform the same procedure without further checks when there is a lack of performance – which can sometimes be incorrect.
A range of other causes for poor performance include, for example, under https://community.dieselschrauber.org/en/viewtopic.php?t=3322.
described.

But even if the workshop is competent enough to read out the mass airflow values during a test drive, it doesn't automatically mean that the MAF sensor is defective if the values are too low.
For example, the following causes are also being considered:
-> Insufficient boost pressure; this should always be checked additionally (using a diagnostic tool or by reading the data).
-> Too low ambient pressure at high altitudes (the boost pressure is intentionally reduced there to protect the turbocharger from overload).
-> Malfunction of the altitude sensor in the engine control unit (ECU), which simulates a high-altitude drive and consequently reduces the turbocharger boost pressure (ambient pressure can also be read).
-> Blocked intake passages / dirty air filter (Caution: Risk of overloading the turbocharger, see LDA technical article).
-> Reduced effectiveness of the intercooler due to heavy contamination, extreme heat, and/or low vehicle speed (indicated by charge air temperatures significantly above 50°C).
-> Leaks between the mass airflow sensor (MAF) and the intake manifold allow the engine to draw in "incorrect air" around the MAF.
-> Engine misfire caused by heavy oil and soot deposits in the intake channels behind the EGR valve.
-> Restricted exhaust pipe diameter due to catalytic converter fragments blocking the passage or crushed pipes after installation or similar incidents.
In such cases, even a perfectly functional LMM (Linear Motion Module) will not resolve the performance issue.
Considering the price of a new linear motor module is around €75 (as a replacement part), it might be worthwhile to investigate potential causes related to this component.

Alternatively, you can borrow a known-good LMM and install it temporarily (without having to buy it immediately) – or try to simulate one.


Diode test

This can be achieved using a common diode (e.g., 1N4148), which is connected to the mass airflow sensor (MAF) in place of the lambda sensor, such that the engine control unit (ECU) constantly receives a signal indicating maximum cylinder filling. As a result, when the accelerator pedal is pressed (fully or partially), the ECU releases the full fuel injection amount.
To do this, connect the cathode (the ring on the housing) to the mass airflow sensor (MAF) signal cable, and connect the other diode terminal to the 5-volt contact (which should be measured against ground with the ignition turned on for safety). When plugging in the diode connector wires, make sure they are securely connected in the plug and do not bend the contact springs. If necessary, secure the diode to the plug with tape to prevent it from coming loose.
More recent engine control units (ECUs) recognize this trick by detecting the implausibility between engine speed, boost pressure, and air mass, and then switch to a power-reduced emergency mode. In that case, of course, this method becomes useless.

However, even in engines with "simple" MSG (Motor Steuergerät) systems, a problem can occur because the mass airflow sensor (MAF) value, which serves as a feedback signal for the EGR (Exhaust Gas Recirculation) control, no longer responds correctly. Consequently, in partial load conditions, the EGR valve opens further than normal, which can lead to increased soot formation and higher fuel consumption.
Solution: Deactivate the EGR system. To do this, disconnect and seal the hose leading to the membrane pump, ensuring that the membrane valve closes properly. If necessary, insert a blanking plate or similar between the exhaust pipe and the EGR valve.

Sure, here is the translation of the text from German to English:

"Note:"
Since a functioning LMM (Lambda Multi Point Sensor) is an essential requirement for meeting emission standards, the replacement of the LMM with a diode invalidates the vehicle's operating permit, regardless of whether the EGR system is also deactivated or not!
Therefore, corresponding test drives may only be carried out outside of public traffic areas!

For example, accelerate with full throttle in 3rd gear from approximately 1300 rpm and use a stopwatch to determine the time between 2000 and 4000 rpm. If, under the same conditions, significantly shorter (or "normal") times are achieved with the diode compared to when the MAF sensor is connected, cleaning or replacing the MAF sensor will usually fix the problem – unless there is an issue with the other electrical connections or connection values for the MAF sensor, or if there is another (more unusual) fault.

If the engine exhibits unusually excessive black smoke during the test drive with the diode acting as a replacement for the mass airflow sensor (MAF), specifically between 2000 and 4000 rpm, it indicates that the engine is receiving significantly too little fresh air. Potential causes include the problems mentioned earlier, which would prevent the engine from achieving its normal performance even with a verschandeln or new MAF sensor.

To gain a deeper understanding of the causes of power loss, it is necessary to read out the injection quantity limitations, torque values, driver demand (throttle position), and air mass using VCDS or a VAG tester, with the MAF sensor connected.
If the vehicle acknowledges the diode as a replacement for the mass airflow sensor (or the deactivated EGR system) with a limp-home mode, then this testing method is the only one that can be considered.

Liegt der Luftmassen-Istwert im AGR-Meßwerteblock beim Vollgasbeschleunigen über 2000 rpm unter dem Sollwert, so muß das noch keinen Leistungsverlust bedeuten. Ein routinemäßiger Tausch des LMM ist dann womöglich rausgeworfenes Geld!
Here, the crucial factor is the measurement data block with torque and air mass limits, as well as driver demand: As long as the opacity limit does not represent the lowest value, replacing the MAF sensor or cleaning it will not result in any performance gain, even if the actual air mass value (especially in the higher RPM range) is below the target value.
Conversely, in engines with high displacement (e.g., ARL: 150 hp / 1.9l), the full-throttle target value for air mass derived from the AGR characteristic map may be too low to raise the opacity limit via torque limitation.

Aber auch eine Trübungsbegrenzung unter dem Drehmoment- bzw. Fahrerwunsch-Wert muß nicht zwangsläufig von einem falsch messenden LMM herrühren (vgl. oben -> Ursachen für Leistungsverlust).

In some TDIs, the torque limitation is not calculated from the mass airflow sensor (MAF) value, but rather from the boost pressure. In this case, a faulty MAF sensor cannot cause a loss of power as long as there is sufficient boost pressure. If in doubt, checking measuring block 8 will indicate whether the maximum possible power is being achieved (if the torque limitation is above the actual torque).


verschandeln the LMM.

If the Mass Air Flow sensor (MAF) has been definitively identified as the cause of the performance issues, an initial attempt to restore its functionality through cleaning may be made in order to save costs.
However, there is no guarantee of success in terms of restoring the engine's full performance, as the mass airflow sensor may also be defective or aged for other reasons at the same time.
In addition, depending on their composition, cleaning agents can damage the delicate sensor membranes.
Cleaning the MAF sensor unnecessarily should therefore be avoided, because replacing a MAF sensor that has been prematurely cleaned is just as expensive as replacing one that is actually defective.
The presence of oil residue and other deposits in the area around the sensor plate can provide an indication of the potential for improvement after cleaning. If a wipe test using a tissue or cotton swab reveals significant buildup in this area (not on the sensor plate itself!), then a still-functional mass airflow sensor should produce better readings after cleaning.
To remove the original screws of the Bosch mass airflow sensor insert, a 20mm Torx tool is usually required. However, even this type of screw head apparently isn't robust enough for DIY use (for whatever reason), so newer cars use different screws again - similar to a Torx with 5 instead of 6 teeth .

If the cleaning fluid (usually brake cleaner spray) drips out looking dark and cloudy, you can be sure that the cleaning was necessary.


Affordable LMM replacement?

-> Using the diode described above as a replacement for the mass airflow sensor (MAF), it is possible to move the vehicle, even over longer distances ("illegally", see above), if the MAF is defective. This allows, for example, to complete a weekend trip with the usual performance. While there is no immediate risk of damage, the potentially higher soot content in the exhaust gases can, over time, make the VTG (variable turbine geometry) more difficult to operate and lead to problems with boost pressure.
If the engine control unit (ECU) goes into a failsafe mode, it may still provide more power than with a faulty mass airflow sensor (MAF), so it's worth trying in individual cases.

-> Ein alternder LMM, der (auch nach einer Reinigung) "nur" zu niedrige Werte liefert, lässt sich mit einer zusätzlichen Öffnung hinter dem Sensorplättchen zwecks Erhöhung der Luftströmung am Sensor wieder hochpäppeln, um den fälligen Ersatz hinauszuzögern (vgl. Anhang).
When drilling near the MAF sensor, work extremely carefully to ensure that the drill bit does not damage the sensing element (use a depth stop or similar)! Alternatively, the rear panel behind the air intake opening can be carefully cut out (e.g., using a saw blade held perpendicular to the airflow direction).
First, create an opening of approximately 2 mm, reinstall the mass airflow sensor (MAF), and check the effect using VAG-COM. To do this, access the AGR measurement block in the basic settings mode. When the EGR is inactive (switching between active and inactive every few seconds), the air mass value for the 1.9L engine should be between approximately 450 and 500 mg/stroke at idle.
If this area is not reached, increase the opening.
This test naturally requires a completely functional AGR system!
The LMM signal should ideally not exceed the values of the original VAG part (e.g., by checking the results through test drives using VCDS (VAG-COM) and comparing them to the target values). This helps avoid side effects, such as those that can occur when using a diode as an LMM replacement, which could jeopardize passing the next emissions test and, over time, lead to increased soot, oil, and sludge deposits in the intake area due to a higher EGR rate.
If necessary, adjust the EGR rate via adaptation to higher air mass values (resulting in a lower exhaust gas proportion).
Attention: In TDIs with an electric throttle valve, the diode test cannot be applied because the engine control unit attempts to achieve the exhaust gas recirculation rate by closing the throttle valve.

-> As a replacement for defective Bosch TDI mass airflow sensors (MAF), a visually similar MAF unit from DC is sometimes mentioned (around €70), which can be screwed into the (reused) MAF housing instead of the original VAG part.
However, previous experiences suggest a different characteristic curve for the aftermarket DC-MAF sensor, which results in a loss of power, especially in the lower RPM range, compared to a new original VAG-MAF sensor.
Possibly the DC-LMM could be trimmed to have a similar characteristic curve as the original VAG-LMM by increasing the airflow at the sensor plate (using the same method as with an aging original LMM); however, there is currently no confirmed practical experience to support this.
Even minor problems that can be easily resolved are reported in connection with the connector design.


The old, one-piece Pierburg mass airflow sensor can be "rejuvenated" by soldering in a resistor to compensate for the decreasing output voltage due to age -> https://community.dieselschrauber.org/en/viewtopic.php?t=4258