Mass Air Flow Sensor (MAF): Expensive Weak Point in VAG TDI Vehicles (Articles)

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

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

The LMM (Lambda Measuring Module) provides an important signal for calculating the fuel injection quantity, which is the mass of air drawn into the engine. Air mass measurement, on the other hand, is only used in some older diesel models. The air flow meter used for measuring air volume operates on a fundamentally different principle than an air mass meter, and is not the subject of this article.

The more detailed relationships are shown in .

For the operation of the TDI, a statement about the fresh air mass per cylinder filling is required (unit of measurement: mg/H).
However, the mass airflow sensor (MAF) only measures the airflow towards the engine, which can be the same at low engine speeds and high boost pressure as it is at higher engine speeds and lower boost pressure.
The MSG therefore divides the mass airflow (LMM) signal (simply put) by the engine speed and calculates the amount of fresh air available per intake stroke in the cylinder.


The mass airflow sensor (MAF) voltage also serves as a feedback signal for the EGR (Exhaust Gas Recirculation) 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 mass airflow sensor (MAF) signal, as defined by the EGR characteristic 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 mass of air being drawn into the engine.

The operating principle of the hot-film mass flow sensor (MFM) was developed further from the earlier hot-wire MFM.
TDI mass airflow sensors essentially consist of thin, temperature-sensitive sensor membranes on a small electrical "heater" in the form of a plate, which is exposed to the intake air.
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 sensors) used until approximately 1998 and the Bosch MAF sensors used subsequently 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 flow direction, which is advantageous for the precise measurement of actual airflow 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, so 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 pins used
1 = Power 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 pins used, analog output signal
2 = Power + 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

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 the highest possible impedance), the signal cable must be tapped.
The analog signal voltage relative to ground with the engine idling is approximately 0.3 volts for the Pierburg mass airflow sensor (MAF), and approximately 1 volt for the 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 deviations in voltage readings (even with an otherwise intact engine and vehicle-side electrical/electronic systems) can indicate contamination and/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's a rectangular signal where the frequency increases proportionally to the measured air mass.

"Does one have a diagnostic system, e.g.,..." With VCDS (VAG-COM) 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 sensors) is already "cleaned" of temperature and pressure influences by internal compensation circuits. Therefore, it reports to the engine control unit (ECU) the amount of air mass flowing through the MAF sensor per unit of time, in the direction of the engine.
To enable rapid adjustment of the injection quantity and EGR rate during changes in engine speed and load, a correspondingly short response time of the mass airflow sensor (MAF) signal to changes in mass flow is required. This means that the sensor membranes must be able to react as quickly as possible to the "new" temperature.
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 airflow. For the EGR control system, this appears as if the EGR valve is already partially open." As a result, the AGR rate will be reduced by the amount by which the LMM underreports.
Since it appears that less air mass is available for combustion in the upper load range, the injection quantity is reduced to correspond to the decreased mass airflow rate. The result is a loss of engine power, which regularly brings TDIs to workshops, where they are often fitted with a new mass airflow sensor (MAF) at a hefty price – provided the root cause is correctly identified.

Attention: Defective mass airflow sensors (MAF 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 (MAF) signal to ground or to the power supply.

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


Common causes of mass airflow sensor (MAF) 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. Coatings are typically formed...
-> 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 rain (especially in the spray from 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 can cause further damage.

In addition, the mass flow meter itself ages, which becomes noticeable over time through decreasing readings for the same mass flow rate.
The result is a (usually gradual) loss of engine performance.
Often, after a mileage of around 100,000 km, the catalytic converters become 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, the problem is frequently resolved by simply replacing the MAF sensor.
Unfortunately, this rule of thumb often leads workshops to do exactly this – to simply state a lack of performance without further checks, which can sometimes be incorrect.
A range of other causes for poor performance include, for example,
described.

But even if the workshop is so competent that it can read the mass airflow values during a test drive, it doesn't automatically mean that the mass airflow 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 boost pressure gauge 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).
-> Defect in the altitude sensor within the engine control unit (ECU), causing a simulated high-altitude condition, which in turn 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 (detectable 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, bypassing the MAF sensor."
-> 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.
In such cases, even a perfectly functional LMM (linear mixing valve) will not resolve the performance issue.
Considering the price of a new LMM (linear motor module) is around 75 euros (as a replacement part), it might be worth investigating these potential causes.

Alternatively, you can try borrowing a known-good LMM and installing it (without having to buy it immediately) - or attempt 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 MAF's extension cable, in such a way that the engine control unit (ECU) constantly receives a signal indicating a maximum cylinder fill. As a result, when the accelerator pedal is pressed (fully), the ECU releases the full 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). Make sure the diode connection wires are securely inserted into the contacts in the plug, and do not bend the contact springs. If necessary, secure the diode to the plug with tape to prevent it from falling out.
More recent engine control units (ECUs) recognize this trick by identifying the implausibility between engine speed, boost pressure, and air mass, and then switch to a power-reduced emergency mode. Then this method is obviously pointless.

However, even in engines with "simple" ECUs, a problem can occur because the mass airflow sensor (MAF) value, which serves as a feedback signal for the EGR control, no longer responds. Therefore, in the partial load range, the EGR valve will open further than usual, which can lead to increased soot formation and higher fuel consumption.
Solution: Deactivate the EGR system. To do this, disconnect the hose leading to the membrane control unit and seal it, ensuring that the membrane valve closes properly. If necessary, insert a blanking plate or similar item between the exhaust pipe and the EGR valve.

Sure, here's the translation:

"Note:"
Since a properly functioning LMM (Lambda Multi-Point Measurement) is a crucial requirement for meeting emission standards, replacing it with a diode invalidates the vehicle's operating permit, regardless of whether the EGR (Exhaust Gas Recirculation) 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, starting around 1300 rpm, and use a stopwatch to determine the time it takes to reach from 2000 to 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 are other electrical connection issues. The connection values for the LMM are not correct, or there is another (more unusual) error.

If the engine exhibits unusually excessive black smoke during the test drive with the diode acting as a replacement for the mass airflow sensor, specifically between 2000 and 4000 rpm, it indicates that the engine is receiving significantly too little fresh air. Possible causes include the problems already mentioned, which will prevent the engine from reaching its normal performance even with a clean/new mass airflow sensor.

To gain a deeper understanding of the causes of performance loss, it is necessary to read the torque limitation values, driver demand (throttle position), and air mass using VCDS or a VAG tester, with the mass airflow sensor (MAF) 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.

If the actual value of the air mass measured in the air mass measurement block exceeds 2000 rpm during full-throttle acceleration but is below the target value, this does not necessarily indicate a loss of power. Replacing the mass airflow sensor (MAF) as a routine maintenance procedure might be a waste of money!
Here, the crucial factor is the measurement data block, which includes torque, driver demand, and air mass limits: As long as the opacity limit is not at its lowest value, replacing the mass airflow 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 from the AGR-related measurement block may be too low to raise the torque limit and thus prevent excessive emissions.

But also a limit on the blurring effect below the torque or The driver's desired performance may not necessarily be caused by a mass airflow sensor (MAF) that is incorrectly measuring airflow (see above -> causes of power loss).

In some TDIs, the smoke limit is not calculated from the mass airflow sensor (MAF) value, but from the boost pressure. Here, a weak LMM (Lambda Measuring Module) cannot cause a loss of performance, as long as there is sufficient boost pressure. If in doubt, a look at the diagnostic data (MWB will indicate whether the maximum possible power is being achieved (if the opacity limit is above the torque).


verschandeln LMM:evil:

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 restoring full engine 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.
Cleanings should generally be avoided unless there is a clear indication of a problem, because replacing a Mass Air Flow (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 a significant amount of residue in this area (not on the sensor plate itself!), then a still-functional mass airflow sensor should provide better readings after cleaning.
To remove the original screws of the Bosch mass airflow sensor housing, a 20mm Torx tool is usually required. Since even this type of screw head apparently isn't secure enough for DIY projects (for whatever reasons), newer cars use different screws again - similar to a Torx screw with 5 instead of 6 points .

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 MAF sensor, the vehicle can be driven for longer distances even with a defective MAF sensor ("illegally", see above), for example, to complete a weekend trip with the usual performance. While there is no immediate risk of failure, the potentially higher soot content in the exhaust gases could, over time, make the VTG (variable geometry turbocharger) 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.

-> An aging LMM (Laminar Mass Flow Meter) that delivers "only" low values (even after cleaning) can be temporarily revived by adding an additional opening behind the sensor plate to increase airflow around the sensor, thereby postponing the necessary replacement (see). (Appendix).
When drilling the mass airflow sensor (MAF), work extremely carefully to ensure that the drill bit does not damage the sensor element (use a depth stop, etc.). Alternatively, the back 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 valve 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 cannot be reached, increase the opening.
This test naturally requires a completely functional AGR system!
The LMM signal should not exceed the values of the original VAG part (e.g., verify the result through test drives using VCDS (VAG-COM) and compare it with the target values). This approach can help avoid side effects, such as those that can occur with a diode as a replacement for the LMM (Lambda Measuring Module), which could jeopardize the next vehicle inspection and, over time, lead to increased soot, oil, and sludge deposits in the intake area due to a higher EGR (Exhaust Gas Recirculation) rate.
If necessary, adjust the AGR (Abgasrückführung) rate via adaptation to higher air mass values, which results 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 sensors), a visually similar MAF sensor from DC is sometimes mentioned (around 70 euros). This DC sensor is then screwed into the (reused) MAF housing instead of the original VAG part."
However, previous experiences suggest a different characteristic curve for the DC-LMM, which, as a result, leads to a loss of power, especially in the lower speed range, compared to a new, original VAG-LMM.
Possibly https://community.dieselschrauber.org/en/viewtopic.php?t=8670the DC-LMM could https://community.dieselschrauber.org/en/viewtopic.php?t=3322be adjusted to have a similar characteristic curve to 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 (= easily solvable) issues are reported regarding the connector design.


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