Hello and welcome to JustAnswer BMW,My name is XXXXX XXXXX it will be my pleasure to assist you today..
How many miles on the car?
I suggest you check fuel pressure to make sure it is adequate (50psi).
Also this sounds like it might be a clogged catalytic converter - so this is another avenue to pursue.
Also, if you are able to have a diagnostic done that can read live data that would also be very helpful in helping to run down any vaules that are out of alignment.
I hope that is helpful. Please let me know if I can be of any further assistance, or clarify anything for you.
It was my pleasure to help today, and I wish you the best with your repair and with your vehicle going forward.
A blocked cat can sometimes be cleared out. You can also install aftermarket cats for not that much (few hundred).
Fuel pressue would be either the pressure regulator or fuel pump, neither is very expensive.
If its a valve problem it would be considerably more expensive - 800 to 1000 I would estimate.
Please let me know if you have further questions.