E39 MAF testing
The MAFs are vital to the proper function of the S62 engine. The engine can never work better than its MAFs.
The MAFs (Mass AirFlow meters) are located in the M5 and Z8 air intake tubes. The MAFs report to the Engine Electronics (DME aka ECU) the amount of air that is drawn into the engine, and what temperature this air is. The DME uses this to calculate how much fuel to inject.
Vital as the MAFs are, those have to be really bad before you actually get a stored fault-code.
The functional test by the BMW tester just tests the MAFs at idle with the car standing still. In my experience that says absolutely nothing about how they perform at WOT (Wide Open Throttle).
I think there is an economical (warranty) reason why BMW does not want the MAF fault code to trigger as soon as the MAFs start to be less-than-perfect, rather than only when there is extremely poor MAF function.
MAFs typically last 50000 miles before getting so contaminated it affects S62 engine performance.
Follow those instructions for “Cluster test functions“, and select the mode for fuel consumption per hour.
With the engine fully warmed up, drive several WOT (wide open throttle) accelerations. Notice the fuel flow per hour numbers. If everything is ok the numbers should increase continuously to about 140 litres per hour at WOT at 7000 RPM. (don’t worry about bouncing into the rev-limiter)
The fuel-flow is directly proportional to the MAF reading reported to the Engine Electronics, so if the Fuel-flow is correct, the MAFs are most likely correct too.