Hi, mine's a 520i with M54 engine. Before, I had a problem when the car is running with Normal automatic mode. The throttle seems to judder somehere right below the 2000 rpm, probably around 1800-2000 to be more precise. By judder I means it was like as if you run slow using too high of a gear in manual.
I had it scanned and it came out that there is a fault with my MAF sensor. The original MAF sensor was VDO, I replaced it with Continental.
After the MAF replaced the engine run ok, but lately I notice that the judder symptom is back but in a different way. Now the judder seems to happen when the engine experiencing a sudden high load, like when you are turning on an intersection and wants to go out as quickly as possible; or when I do a little kick down on the throttle it seems like the engine is "confused" whether it should downshift or just open up the throttle.
Anyone can help me?

 

Perform a smoke test

 

By smoke test do you mean like emission test?

 

A smoke test is to find air leckage in your intake system. If you have a diagnostic system you can check the long-term and short-trim values instead. These are injection time correction values and should be close to "0". If there is a leak (= more air in the engine than the MAF is detecting the values will be positive). A modern vehicle can tolerate a small leakage (about 20%) but not too much (mostly it's CCV or intake boot leakage). Could be the automatic transmission also (or both). You should check the status values (pressure and time). It's possible with Tool32.

Quick and easy test: To test the MAF (AND LEAKAGE!) just disconnect the MAF. Your engine will run on a good enough replacement map in open loop (without MAF sensor signal, so it doesn't care about leakage after MAF anymore). If it runs better it's either MAF or leakage. You can run it longer without MAF. You will get some more fuel consumption, though.