There's a common problem with the BMW M50 engines as fitted to the E36 320i and 325i, and the E34 520i and 525i.
On the intake manifold there is a air pipe connection that over time can come loose. Then, usually when starting the car, this connection can blow off resulting in a large vacuum leak. The car will be reluctant to start and run, idle badly, and have no power brakes.
The fix is to replace this connector and o-ring, but for a temporary fix you can usually just plug it back in. On my 325i the previous owner had ran a cable tie around the intake manifold to secure this connector in place. I suffered the problem twice in only a few months but didn't get around to replacing the part before I sold the car.
The connector is located on the engine side/underside of the intake manifold almost opposite of where the air intake goes into the manifold. The connector is held in place by a clip only on one side.
For replacement the part numbers are below. Note that there are different part numbers for pre and post February 1992 models. My car was a 1991 model and I don't know if the problem effects the post February 1992 models, or possibly even the M52 engine too. The newer part looks the same except for the end that goes into the manifold, so presumably, it has been improved, but I do not know if they are interchangeable. Do you know? Please leave a comment.
Connector, up to 02/1992 11611730265
Connector, from 02/1992 11611738629
O-ring, 24×3.5 11611730725