One causes of turbo failure on the Bmw 320d or Bmw 120d is oil carbonisation. There are several theory's for oil carbonisation but lets look at for what it is, solidified lumps of oil.
One of the main causes of oil carbonisation is injector blow pass, this is where the actual
combustion gases blow past the injector washers which seal the fuel injectors in there locations.
This blow pass gas can make its way into the engine oil system and when the oil cools down.
we get what is called carbon lumps.
The Bmw 320d and Bmw 120d are fitted with crankcase breather filter which vent crankcase gases back into the engines induction system, if the filter blocks, then the
crankcase gases can conaminate the oil and causes oil dilution, once this happens then the turbo on your Bmw 320d or Bmw 120d will begin to fail.
The first sign of turbo failure is turbo whistle , this is normally followed by a drop in performance and smoke form the exhuast.
Now that you know the cause of your Bmw turbo failure lets look at some prevention methods you can put into practice once you have got the turbo
repaired on your BMW. The main thing you need to address is the engine oil,(ie) the condition of it and how long its been in there. BMW recommend using
long life oil which is not due to be changed at about 12,000 mile intervals, the problem with this is the oil is constantly getting bombarded with
crankcase gases creating soot particles and leading to sludge, the only way to prevent this is to ignore the long service intervals and change the oil and filter every 6000 miles.