Hello, my 2007 automatic VW Passat 2.0 TDI is not raising to 40.

Hello, my 2007 automatic transmission and petrol VW Passat 2.0 TDI is not raising above 30. I have changed the coils and spark plugs. Before there was cloudy thick smoke coming out of the exhaust and when it happens it slows the speed and can't climb a hill. It was carbonized and the problem was fixed but it's still struggling to raise without smoke anymore. Please I need your advice.