My best guess would be a large vacuum leak. The engine computer can probably detect when the air flow meter is disconnected, and switches to a default fuel map based upon the engine RPM and throttle position. The default map is not optimum, but it works well enough to run the engine until it can be serviced. When the air flow meter is connected, the engine computer tries to give the appropriate amount of fuel according to the measured air intake, but the large vacuum leak allows a large amount of air into the engine that is unmetered and consequently the amount of fuel released is insufficient, and the engine runs poorly. A large vacuum leak could because by a rupture in the power brake booster, or a backfire could have blown the hose from the idle control valve off of the intake manifold. On a 6 cylinder BMW, this hose is connected on the underneath side of the intake manifold and is difficult to see. It may be the same on the 4 cylinder engine, so look there.
I'd like to know if this is the right guess, please post your findings.
Dr. Loot's Assistant
Monday, October 26th, 2009 AT 10:10 PM