What causes carbon build-up

Due to modern unburned hydrocarbon regulations, vapors from the crankcase are usually vented into the intake stream in order to prevent oil droplets from escaping through the exhaust. In a port injection traditional engine, these droplets are ‘washed off’ the neck of the intake valve by a relatively constant stream of fuel. In a DI (direct injection) engine, the gasoline doesn’t touch the intake side of the valve. As a result, the droplets have a tendency to bake onto the valve and cause significant carbon deposits causing a reduction in performance, rough idling, difficulty starting and eventually bad sealing of the valves. 

To add to this effect, many advanced DI engines also include exhaust gas recirculation in order to lean out the combustion mixture and reduce in-cylinder temperatures for certain combustion modes (reducing NOx emissions). Since DI combustion has the ability to produce far more soot than premixed combustion (port injection), the problem is magnified.

Even more alarming is that these deposits can dislodge and damage other downstream components (turbochargers, catalytic converters, etc.). Manufacturers have added systems to capture these oil droplets and particulates, but no system is 100% effective. As a result, there are many disappointed early adopters with large repair bills. Cars with as low as 50,000km are affected and benefit from the clean. Even diesel engines haven’t been immune to these issues.