Modern engines are often direct injection, as this process allows for advantages in fuel consumption as compared to conventional fuel injection, as well as yielding more power with an engine of identical displacement. However, the disadvantage of this is that the regular bathing and therefor cleaning of the intake valves by way of the fuel that they are exposed to does not happen any more. The intake valves are only in contact with air or rather blowby gases from the crankcase breathing circuit, in which substantial quantities of fuel and oil can be found. Over time, these lead to deposits of carbonised fuel and oil in the intake tract as well as on the intake valves themselves; this is also referred to as carbonising.
This is not immediately bad for the engine, but over time can have a detrimental effect on its efficiency. If the valves are heavily carbonised, they may not close properly anymore, and symptoms such as a bumpy idle, vibrations and diminished throttle response can be observed; it may also contribute to increased oil consumption.
As unfortunately this carbonising effect is an inevitable byproduct of direct injection, it cannot be prevented. The use of additives in the fuel itself is useless, as (see above) the fuel does not come into contact with the intake tract or the valves at all; the use of water/methanol injection may slow down the carbonising somewhat (depending on where the methanol is injected and provided it is not yet completely vaporised when it reaches the valves), but cannot prevent it either. A cleaning of the intake valves by using Seafoam or similar products which are injected directly into the charge pipe has been discussed extensively, but equally this method is not very efficient as the carbonising is usually too persistent to be removed by this method.