Q&A

Black smoke from the exhaust pipe is a sign of excess fuel. Too much fuel is entering the engine. Many things an cause this. The 1.8L and 2.0L engines in the Jetta this year use totally different fuel injection systems but in either case key inputs to the fuel injection computer such as how much air the computer "thinks" is going into the engine, and what temperature the fuel injection computer "thinks" the engine is are critical inputs. Too much fuel pressure or a leaking fuel injector will cause black smoke, poor fuel economy and rough running. Excess fuel can was lubrication away from the cylinder walls, dillute engine oil and damage the engine.
It is foolish fro me to speculate what is wrong with your car with out being able to drive your car first hand. Get your car checked out by someone familiar with VW's hopefully it is something simple!
Hopefully the smoke out the exhaust tailpipe is not "blue" in color. Blue smoke is a sign of excess engine oil consumption either the engine has too much oil in it or the engine has worn piston rings or valve guides.

Failed engine coolant temp sensors are common on these.
It is blue with four wire beige connector , mounted at coolant hose near battery....

its your head do your self a favour take a paper hold it at your exhaust while the car is idling if the paper gets sucked in than your valves is bend have to take your head of and replace new valves

your car sucks