That is right a Bimmer. Now, I don't normally even touch these cars but I dabble from time to time. This is one of those times one of my good customers has this pretty clean BMW with 181,582 miles on it. Apparently, it had a Check Engine Lamp on and one of his techs pulled a P0171 (System Lean) code. When I get there the codes and more importantly the freeze frame data has been cleared. Since I don't normally diagnose these cars, all I can use is generic scantool data. So I focus in on our fuel trims and MAF (Mass Air Flow) sensor inputs. I know these have common issues with MAF sensors and Ventilation valves causing lean codes. I check fuel trims at an idle and they are pretty tight, raced up they start to drift into the positive side. I figure slam dunk I have a dirty MAF.
Looking at the MAF hot wire it is really clean and the customer has a factory BMW air filter. No cheapy air filter here. So, no slam dunk here. A road test is in order. I monitor short term fuel trim, rpm, and MAF g/s values. I am still thinking bad MAF here. The peak g/s easily achieves our liter displacement times 40 rule of thumb and short term fuel trims do not follow air flow. The one thing I do notice during the test drive is a distinct whistle on wide open throttle. Hmmm. Experience tells me when I hear a whistle it is usually caused by an intake restriction or some type of hole in the air tract. Since I already inspected the air box and it has a quality air filter installed. I head to the air ducting after the MAF sensor.
Bingo! Now I am a BMW expert. No way! A seasoned BMW tech would have probably found this in half the time. Here is another shot below.
The point here is some strategic testing even in generic mode and we can fix cars we are not exactly familiar with. I report my findings to the shop owner and advised him to change that much maligned ventilation valve with the updated one as maintenance considering the mileage on this car. I also advise him of Standard Motor Products new line called TechSmart that my good buddy Joe Donaggio manages. This new line is all previously dealer only, high failure, problem solving parts like these ventilation valves. It is a great alternative to going back to the dealer. I and others give Joe constant feedback from the technical "real" world and he does the leg work to make it happen. Check it out at http://www.techsmartparts.com/.