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Too much air mass on diesels.
On E&F diesel engines of the BMW N47, N57 series, the fault code "mutual air flow reliability, measured air mass is too large" or "the ratio of measured and calculated air mass is too large" is quite common; the wording may differ depending on the control system. Such a malfunction usually does not lead to power limitation, but only to the illumination of the MIL warning light on the instrument panel.
The reasons can be divided into three groups:
1) a malfunction in the air mass measurement (for example, a defective flow meter)
2) malfunction of the recirculation system
3) leakage of the intake tract
Troubleshooting should begin with an assessment of the reliability of the air mass readings when the engine is idling. The average normal air consumption for BMW diesel engines is:
for 4-cylinder М47/N47 about 43-45kg/h*
for 6 cylinders M57/N57 approx. 63-66kg/h
The control must be carried out on a warm internal combustion engine, with consumers turned off (air conditioning, headlights, stove, etc.) and with the EGR system turned off (with the EGR valve chip removed), so that all the air mass enters through the air flow meter. If the measured air mass differs from the specified one at idle, and the situation does not change after a trial replacement of the air flow meter, you can proceed to the diagnosis of the intake tract.
When a BMW diesel engine (and possibly not only) is idling, the turbine creates a slight overpressure in the intake manifold (about 40-60 mbar). If there is a significant leak, the performance of the turbine is sufficient to provide the necessary air flow for the operation of the internal combustion engine and blow out part of the air "outside". Thus, the turbocharger pumps more air mass through the flow meter than the engine receives. Leaks in the case of increased air mass should be looked for after the turbocharger. Leaks upstream of the turbine can lead to a reduced measured air mass and are not considered in this article.
Leakage can be searched for by oil emissions on the elements of the intake tract (for example, damage to the pipe, missing or damaged sealing rings, etc.). Typical leak points for N-series engines: swirl flap shaft seal, EGR heat exchanger bypass channel, intake manifold cracks.
In order to quantify leakage, it is possible to apply forced control of the turbine: In the case of electric control of the guide vane (and these are all single-turbine modern BMW diesel engines), it is necessary to remove the rod connecting the turbine actuator and the lever connected to the guide vane. Next, we start the engine, control the turbine manually and observe the operating parameters. With the maximum activation of the turbine on x.x. an excess pressure of about 150-170 mbar is created in the intake tract, and with minimal activation (the turbine creates resistance to flow) - a slight vacuum of 70-90 mbar. In the case of significant leaks, at maximum activation, we will see an increase in air mass (the turbine blows through the "hole"), and at a minimum - a decrease (the turbine slows down the air flow, and the engine sucks in through the "hole"). With a sealed intake tract, the above manipulations also cause changes in the air mass measurement, but these changes are negligible. You can also use the method of pressure testing the intake tract using a smoke machine.
!In this case, the following situation may occur: the leak is not very significant, or there is a crack (for example in the plastic intake manifold), which opens at high pressures, then the fault may not be detected during the idle test. In such cases, it is advisable to use the logging method. Roll back the VSH logs (a series of accelerations at 100% of the pedal in one gear over the entire rev range) and plot the air consumption characteristic. Next, compare the obtained characteristics with similar ones obtained on obviously serviceable vehicles.
If the malfunction does not appear during the diagnostics, or is sporadic (it does not appear regularly during the operation of the car by the client), you can test replace the EGR valve. When the diesel engine is running at idle and partial loads, the exhaust gas recirculation system is activated. The engine receives part of the air mass through the flow meter; and the rest is exhaust gases supplied to the intake manifold through the USR valve, bypassing the flow meter. If the EGR valve sticks in the closed position, then the proportion of exhaust gases decreases, and the amount of fresh air entering through the flow meter therefore increases. It should be noted that the EGR usually sticks in both closed and open positions, which is accompanied by uneven engine idling and the occurrence of related fault codes ("air mass too small)
Look carefully for defects in the intake manifold, engine leaks (seals, gaskets). It is better to have a Smoke Generator for this.
The intake manifold, AGR heat exchanger, it is better to remove, clean, wash and see the damage. Replace all seals (gaskets) damaged parts and return everything to its place. After that, update the software of the engine (car) to the latest integration level.

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