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.
Here you can see a bearing housing register with fairly extensive corrosion damage.
Corrosion damage like this results in a loss of seal between the bearing housing and turbine housing, and can cause a series of knock on effects, including damage to the bearings, turbine, compressor wheel, and housing.
This kind of damage is really common on Porsche units, because the turbo is positioned so low down on the vehicle.
This means that the turbo ends up catching a lot more wet dirt and road debris thrown up from the back wheels than in other turbos, and over time, this causes corrosion damage to the surfaces of the bearing housing.
Whilst minor corrosion damage can sometimes be repaired, in the majority of cases, the parts will need to be replaced.
Damage to the Seal area
In this image, you can see damage to the bearing housing seal area, caused by excessive play and wear to the journal and thrust bearings.
In a new bearing housing, the seal area would have a smooth, parallel bore, but as the journal and thrust bearings have worn, the excessive play has caused them to wear away at the seal area, causing a step in the housing.
Excessive play in the journal/thrust bearings is almost always caused by an oil problem. The bearing system relies on a constant supply of clean oil to work effectively, and loss of lubrication for even a couple of seconds will cause wear between the surfaces. Low oil pressure, a disruption to the oil supply, leaks and contaminated oil can all cause damage like this.
If this happens to your turbo, the only effective course of action is to replace the bearing housing, along with the worn bearings.