Faulty Bosch MAF Good Bosch MAF
VW uses several Bosch MAFs for its TDIs. Bosch number 0 281 002 216, VW number 028 906 461 and also Bosch number 0 280 217 121, VW number 06A 906 461. These MAFs are equal, except for the length of the housing.
Faulty Bosch Mass Air Flow sensor
The graph below was measured on a 81kW TDI (engine AFN, after 180,000 km) under WOT (wide open throttle). As can be seen clearly, the specified air flow is much higher than the actual (measured) air flow. Turbo pressure reached 2000mbar, so turbo seems to be fine. Also the intake manifold was not fouled by soot and oil.