1991 BMW 850

The sympton you describe can be caused by an entire cylinder bank not functioning. A way to determine if an entire bank is not working is to disconnect the mass air flow sensor while the engine is running. If the engine dies, then that bank is the only one working. If there is no change in the idle when the mass air flow sensor of a bank is disconnected, then that bank is not working. In my opinion, the highest probable cause in such a circumstance is most likely a bad fuel pump. The 750iL has two fuel pumps, one for each bank. If one dies, then the engine will still run, but with very poor acceleration because of the lack of power from one bank. The next most likely cause for the problem you describe would be the complete failure of the ECU for one of the banks, but since this part has no moving components as a fuel pump does, it's less likely to occur.