Mass flow meters (LMM): an expensive weakness in VAG - TDI (Articles)

Air Mass Meter (LMM): An Expensive Weak Point in TDI

The mass air flow meter (MAF sensor, also known as G70) is used for

The LMM provides an important signal for calculating the injection quantity, which is the mass of the air sucked into the engine. Air mass flow measurement is only used in some older diesel models. The flow meter used for measuring air volume functions fundamentally differently from a mass flow meter and is not the subject of this article.

The detailed information can be found at /viewtopic.php?t=8670.

To operate the TDI engine, a statement regarding the fresh air mass per cylinder filling is required (unit of measurement: mg/H).
However, the LMM only measures the airflow in the direction of the engine, which can be the same at low RPM and high boost pressure as at higher RPM and lower boost pressure.
Therefore, the MSG divides the LMM signal (simplified, the engine speed) by the engine speed and thus calculates the available fresh air mass per stroke in the cylinder.


The LMM (Manifold Absolute Pressure) value also serves as a feedback signal for the AGR (Air-Gas Ratio) control: When the AGR is open, a portion of the normally aspirated fresh air is replaced with exhaust gases, which causes the LMM value to decrease accordingly. The AGR is controlled so that, depending on the engine's operating condition (boost pressure, engine speed, and injection quantity), the target value of the LMM signal according to the AGR characteristic curve is achieved.


A few technical facts about the LMM

The LMM is located between the air filter and the turbocharger and measures the amount of air sucked in by the engine.

The operating principle of the TDI hot-film LMM was further developed from the earlier hot-wire LMM.
The TDI-LMMs consist essentially of thin, temperature-sensitive sensor membranes on a small electric "radiator" in the form of a plate that is surrounded by the intake air.
The airflow cools the sensor membrane, thereby changing the electrical resistance value of the membrane.
This change in resistance forms the basis for generating the actual LMM signal voltage through an integrated (analog) amplification and signal processing circuit within the LMM.

The differences between the Pierburg-LMM used until approximately 1998 and the Bosch-LMM subsequently used are mainly due to the electrical connections and voltages at the signal pin.

The newer Bosch development not only detects the main flow but also the flow direction, which is particularly advantageous for measuring highly pulsating air columns (due to the opening and closing rhythm of the inlet valves) at low flow speeds, enabling the precise measurement of the actual air flow.
This necessitates an increased initial voltage without air circulation, so that reverse flows can be reported due to a correspondingly lower voltage.

Air Mass Flow Meter Pin Diagram

Pierburg: Single-piece LMM, 6-pin connector with 5 occupied pins
1 = Power supply + 5 Volts
2 = Mass (Sensor String)
3 = Power + 12 Volt
5 = Mass (Battery / Chassis)
6 = LMM Signal

Bosch HFM 5: two-part LMM (tube and sensor insert), 5-pin connector with 4 occupied pins, analog output signal
2 = Power + 12 Volt
3 = Mass (Sensor String)
4 = Power + 5 Volts
5 = LMM Signal

Bosch HFM 6: two-part LMM (tube and sensor insert), 5- or 4-pole connector with 4 occupied pins, analog or digital output signal
1 = Power + 12 Volt
2 = Mass (Sensor String)
3 = Power + 5 Volts
4 = LMM Signal

or

1 = Power + 12 Volt
2 = Mass (Sensor String)
3 = free
4 = Power + 5 Volts
5 = LMM Signal

The pin numbering is directly visible on the inside of the LMM, for the Bosch LMM, pin 1 is located on the rounded side of the connector.

To directly measure the LMM signal with a multimeter (preferably with a high impedance), the signal cable must be tapped.
The analog signal voltage relative to ground when the engine is idling is approximately 0.3 volts for the Pierburg LMM and approximately 1 volt for the Bosch LMM.
When idling, the voltages are approximately 1 volt higher without the AGR (Gas-Rückführungs-Anlage) system.
The voltage increases with both RPM and boost pressure, reaching a maximum of approximately 4.5 volts (for both types).
Strongly deviating values (despite otherwise intact engine and vehicle electrical/electronic systems!) indicate contamination and/or defects in the O2 sensor.

When dealing with sensors with a digital output signal, testing is only possible using an oscilloscope, or a better diagnostic system. It is a rectangular signal where the frequency is proportional to the measured air mass.

Does one have a diagnostic system, e.g. With VCDS (VAG-COM) available, you can directly read and compare the corresponding measuring blocks of the engine control unit to see if the actual value of the measured air mass is at least equal to the target value.

The initial signal from all LMMs is already "cleaned" of temperature and pressure influences through internal compensation circuits, and therefore reports to the Engine Control Unit (ECU) how much air mass per unit of time flows from the LMM towards the engine.
To enable the most rapid adjustment of the injection quantity and EGR rate during changes in engine speed and load, a correspondingly short response time of the LMM signal to changes in mass flow is required, i.e., the sensor membranes must be able to switch to the "new" temperature as quickly as possible.
This requires a minimal own heat storage capacity, which is achieved through tiny sensor plates and layer thicknesses of a few thousandths of millimeters.
As a result of the minimal layer thickness, the LMM sensor plates are highly sensitive to any contact with solid bodies, which usually leads to damage to the membranes and renders the LMM unusable.


When the LMM (Linear Mass Airflow Sensor) is faulty, it usually reports a value that is too low. This appears as if the AGR (Exhaust Gas Recirculation) system is already partially open. Consequently, the AGR rate will be reduced by the amount that the LMM under-reports.
Since there also appears to be less air mass available for combustion in the upper load range, the injection quantity is reduced to match the reduced value of the air mass meter (LMM). The result is a loss of engine power, which regularly sends TDI engines to workshops, where they are usually fitted with a new LMM at a hefty price – provided the cause is correctly identified.

Warning: Defective ABS sensors with very low measured values do not generally cause an entry in the fault memory, even if the driver perceives clear errors (e.g., significant loss of power, possibly wave-like acceleration . . . )!
Only in cases of severe electrical faults (the LMM signal is shorted to ground or to the supply voltage, etc.) is an error recorded.

Depending on the size of the LMM measurement error, the fuel consumption will also increase: The boost pressure will continue to be regulated to (theoretically) verschandeln combustion of the injection amount requested by the accelerator pedal, while the actual injection amount can be significantly lower due to the defective LMM.
D.h. the boost pressure is (measured against the actual fuel injection amount) increased too much, which unnecessarily consumes energy -> the fuel consumption increases.


Common causes of LMM defects

"Harsh" vibrations (caused by construction defects in the vehicle or incorrect installation of the LMM) can subject the sensor plates to such stress that they break, causing the LMM to fail suddenly and completely.

Any coatings on the membrane will reduce the LMM measurement value, as they impede the heat transfer from the sensor to the flowing air. Belays typically occur
-> through oil fumes from the crankcase ventilation, which spread from the crankcase to the intake tract and eventually to the MAF sensor after the engine has been turned off "in reverse"
-> through fine foreign particles (dust) and other fumes that pass through the air filter
-> through water, which can enter the air filter during rainy rides (especially in the spray from the driver) and can also permanently damage the LMM after some time - especially in winter, when aggressive de-icing salt is dissolved in the spray.

Additionally, the LMM ages over time, which is also noticeable in the form of decreasing measurements at the same mass flow rate over its operating life.
The result is a (mostly gradual) decrease in engine performance.
Often, LMM (Linear Motion Motors) are so dirty or worn after a run of 100,000 km that the motor noticeably loses power.


Since the LMM is often the sole cause of performance loss, the problem is usually solved by replacing the LMM accordingly.
This guideline unfortunately often leads workshops to do exactly this without further checks, even when there is a lack of performance - which can sometimes be incorrect.
A possible cause for performance issues is, for example, found at /viewtopic.php?t=3322
described.

But even if the workshop is so competent at reading air mass values during a test drive, it doesn't necessarily mean that the MAF sensor is defective if the values are too low.
For example, the following causes should also be considered :
-> Insufficient loading pressure, this should always be checked additionally (using LDA or reading values)
-> Low ambient pressure at high altitudes (where the boost pressure is intentionally reduced to protect the turbocharger from overstress)
-> Defect of the altitude sensor in the MSG, which simulates a high-altitude trip and consequently reduces the boost pressure (the ambient pressure can also be read out)
-> Clogged intake passages / Dirty air filter (Caution: Risk of overloading the turbocharger, see LDA technical article)
-> reduced effectiveness of the LLK due to strong pollution, extreme heat, and/or low driving speed (indicated by intake air temperatures significantly above 50°C)
-> Leaks between the LMM and the air intake system cause the engine to suck in "incorrect air" past the LMM.
-> Engine breathing difficulties due to heavy oil soot buildup in the intake passages behind the AGR valve
-> Narrow exhaust passage due to broken catalytic converter fragments or crushed pipes after installation, etc.
In such cases, a completely intact LMM will, of course, not fix the performance loss either.
Given the prices for a new LMM at 75 euros (as a replacement part), therefore, investigating in this direction could be worthwhile.

Alternatively, you can borrow and install a safe and intact LMM for testing purposes (without having to buy it immediately) - or try to simulate one:


Diode Test

This can be done using a universal diode (e.g., 1N4148), which is connected instead of the LMM in place of its Strecker connection, so that the MSG is constantly informed of a maximum cylinder fill level, and this allows it to release the full injection amount when (full) throttle is applied.
To do this, the cathode (= ring on the housing) is connected to the LMM signal cable, and the other diode connection is connected to the 5-volt contact (which should be checked against ground first, for safety, when the ignition is switched on). When inserting the diode connection wires, make sure they make a secure contact with the connector and do not bend the contact springs. If necessary, secure the diode to the connector with tape to prevent it from falling off.
Newer MSG can detect this trick by identifying the implausibility between RPM, boost pressure, and air mass, and subsequently switch to a reduced-power, emergency mode. Then this method is, of course, pointless.

However, even with engines that have "faulty" MSGs, a problem can occur because the LMM value is no longer responding as a feedback signal for the AGR control system. Therefore, in the partial load range, the AGR valve will open further than normal, which can lead to increased smoking and increased fuel consumption.
Solution: Shut down the AGR. To do this, disconnect the hose from the membrane reservoir and close it, ensuring that the membrane valve closes properly. In case of emergency, use a cover plate or similar between the exhaust pipe and the AGR valve.

Note:
Because a functioning LMM is a crucial requirement for complying with emission standards, the vehicle's operating permit is automatically revoked due to the diode replacement, regardless of whether the AGR is also deactivated or not!
Therefore, test drives must only be carried out outside the public road area!

For example, in 3rd gear, accelerate from around 1300 rpm with full throttle, and use a stopwatch to determine the time between 2000 and 4000 rpm. If significantly shorter (or "normal") times are achieved under the same conditions with the diode compared to with the attached O2 sensor, then cleaning/replacing the O2 sensor will usually fix the error - unless the other electrical connections/ Connection values for the LMM are not correct, or there is another (exotic) error.

If the engine is excessively rich (too much fuel) when using the diode as a replacement for the O2 sensor during the aforementioned test run between 2000 and 4000 rpm, the engine is receiving significantly less fresh air (mass). Possible causes include, among others, the previously mentioned problems, which will prevent the achievement of normal performance even with a clean/new LMM.

To gain a deeper understanding of the causes of performance loss, it is necessary to read out the injection quantity limits, torque, driver demand (throttle position), and air mass with VCDS or a VAG-tester when the mass airflow sensor (MAF) is connected.
If the vehicle acknowledges the diode as a replacement for the O2 sensor (or the deactivated EGR) with a fault code, then only this method of testing is applicable.

If the actual air mass value is below the target value in the AGR measurement block during full throttle acceleration above 2000 rpm, this does not necessarily mean there is a loss of power. A routine replacement of the LMM might then be a wasted expense!
"Here, the sensor block with the fuel injection volume limits, torque, driver demand, and air mass is the key factor: As long as the sensor limit does not represent the lowest value, replacing the O2 sensor or cleaning it will not result in any performance gain, even if the actual air mass value (especially at high engine speeds) is below the target value.
Conversely, engines with high power-to-liter ratio (e.g. ARL: 150 PS / 1,9l) – the full throttle target value for air mass from the EGR valve is too low to overcome the opacity limit beyond the torque limit.

But also a torque limitation under the torque/ "The value indicating the driver's desire does not necessarily have to originate from a faulty lambda sensor (see above -> causes of performance loss)."

In some TDIs, the boost limit is not calculated from the LMM value, but from the boost pressure. Here, a weak LMM cannot cause a power loss, as long as there is sufficient boost pressure. In case of doubt, a look at the MWB 8 will provide information on whether the maximum possible power is being achieved (since, if the turbidity limit is above the torque, it is).


verschandeln LMM

If the LMM is definitively identified as the cause of the performance issues, an initial attempt can be made to restore its functionality by cleaning it, in order to save costs.
However, there is no guarantee of success in the sense of restoring the full engine performance, as the LMM may also be defective or aged due to other factors.
Additionally, cleaning agents can, depending on their composition, damage sensitive sensor membranes.
Therefore, cleaning only on suspicion is better avoided, because replacing a filter that has been cleaned preventatively is just as expensive as replacing a "defective" filter.
Evidence of the potential success of a cleaning can be found in the presence of oil fumes and other deposits around the sensor plate: If a wipe test with a tissue, cotton swab, etc. (NOT on the sensor plate itself!!) reveals a significant amount of residue, the sensor will likely provide better readings after cleaning.
To remove the original screws of the Bosch LMM insert, a 20mm hole Torx tool is usually required. Even this screwdriver head apparently isn't robust enough for assembly (for whatever reason), so newer carriages use different screws - similar to a 5-point Torx with 5 instead of 6 points .

If the cleaning fluid (usually brake cleaner spray) is dripping out dark and cloudy, it can be safely assumed that the cleaning was necessary.


Affordable LMM Replacement?

-> Using the diode described above as a substitute for the O2 sensor, the vehicle can also be driven for longer distances ("illegal", see above) without issue, for example, to complete a weekend vacation trip with the usual performance. There is no immediate danger of failure, however, the potential for a higher soot content in the exhaust gas could, over time, make the VTG (Variable Torque Converter) more difficult to operate and lead to problems with the turbocharger.
If the MSG goes into emergency mode, there is still potentially more power available than with the faulty LMM, so it's worth trying in individual cases.

-> An aging LMM that (even after cleaning) only provides "low" values, can be revived by adding an additional opening behind the sensor plate to increase airflow to the sensor, in order to delay the necessary replacement (see: Appendix).
When drilling into the LMM, work extremely carefully so that the drill does not damage the sensor plate (e.g., using a depth stop). Alternatively, the back wall behind the air inlet opening can be carefully sawn out (e.g., using a saw blade held perpendicular to the flow direction).
First, create an opening of approximately 2 mm, then reinstall the LMM, and primarily check the effect using VAG-COM. To do this, essentially, access the AGR measurement block in the basic settings mode. A non-active AGR (the status switches between active and inactive every few seconds) should result in a mass flow value of approximately 450 to 500 mg/cycle in the 1.9-liter engine at idle.
If this area is not reached, increase the opening.
This test naturally requires a completely intact AGR system!
The LMM signal should not exceed the values of the original VAG part (the result should be checked, for example, through test drives with VCDS (VAG-COM) and compared with the target values). This can help avoid side effects similar to those seen with a diode as a substitute for the LMM, which could jeopardize the next MOT test and, over time, lead to more soot-oil sludge deposits in the intake area due to a higher AGR rate.
In case of emergency, adjust the AGR (Air-Fuel Ratio) rate to higher air mass values = lower emissions.
Warning: When performing a diode test on TDIs with electronic throttle bodies, the diode test cannot be performed because the engine control unit attempts to achieve the exhaust backflow rate by closing the throttle body.

-> As a replacement for defective Bosch TDI LMMs, DC occasionally recommends a practically identical LMM insert (around €70), which is screwed into the (widely used) LMM pipe instead of the VAG part.
However, previous experiences suggest that the DC-LMM has a different characteristic curve, which, in particular, results in lower performance compared to a new original VAG LMM, especially at lower engine speeds.
Potentially, the DC-LMM could be tuned to a similar characteristic as the original VAG-LMM by increasing the airflow at the sensor plate (same method as with the aging original LMM); however, there is currently no secured practical experience.
Easy-to-solve problems are also attributed to the shape of the connector.


The old, single-piece Pierburg LMM can be "revived" due to age-related decreasing output voltage by soldering in a resistor -> /viewtopic.php?t=4258