To tell you the truth its a Diesel engine, so I dont know much about it, But I do Know that diesel have turbos for a reason.. To get going easier.. So the turbo MIGHT be the problem..
So I dont agree or disagree with anyones statements.. I would have the dealership do the work, And if it doesnt fix the problem, but first make sure they will reimbuse you if thats not the problem.. also By telling them that you might show that you know a thing or 2 and MAYBE they might actually do a correct diagnosis.. The dealership your going to is a BMW dealership right?
Im not a BMW dealership, but If I fix someones car and it doesnt work right I continue to work on it until its fixed, and free of any additional charges.. I know not all shops are like this, but thats how I work!
**deleted** We do not allow commercial activity from non supporting vendors.