Ahh yes...the ever popular P0171, P0174 and P0313 codes. Welcome to the M54 world

The majority of the time these are related to vacuum leaks. If you smell oil then it could be the valve cover gasket. Usually you can notice it on the front of the engine on the passenger side.

The rubber intake boot is a very popular source of vacuum leaks. The accordion part of the boot is prone to cracking and can cause all kinds of problems.

However the best way to know for sure is smoke test it like you mentioned.

Anything gasket/rubber hose on these cars basically need replacing.