Failure from excessive exhaust temperatures or hot shutdown of engine results in carbon build-up. It is recommended that you idle the engine for two to three minutes to cool the bearing system before shutting down. Turbine end heat soak into the bearing housing results in oil carbonisation and corrosion of the bearing system.
The main damage occurs to the shaft seal ring and grooves, turbine end bearing and bearing housing oil drain cavity blockage.
Carbon build-up can be caused by:
Hot shutdown of engine
Degraded oil quality carbonising in service
Infrequent oil change intervals causing oil breakdown in service
Air and gas leaks
Faulty fuel injector pump/injectors.