In this example, you can see that the bearing housing galleries have become completely blocked and choked up with carbonised oil.
Whilst build up of oil in the bearing housing like this has a number of causes, the most common are a lack of regular servicing, infrequent oil changes, overheating of oil, and engine breather problems that cause oil to be held in the bearing housing as opposed to flowing through it.
In addition to affecting the efficiency and output of the turbocharger, carbonised oil build up will also cause wear damage, as the oil bakes solid, and gets stuck in the housing. In turn, this prevents clean oil from flowing to the bearings, reducing lubrication and increasing damage from wear and erosion.
If this problem is caught early enough, it can be solved by carefully cleaning the bearing housing and removing the blockage, but in some cases, the resulting wear will mean that the housing and bearings need to be replaced.