applications. Excessive crankcase pressure won't only occur due to blow-by gasses escaping past the piston rings it can also be introduced when positive pressure from the intake manifold makes its way into the crankcase. As previously mentioned, in vehicles with forced induction systems such as turbochargers or superchargers, the engine's intake manifold experiences positive pressure under load. This differs from naturally aspirated applications where the intake manifold will remain in vacuum while under load. Thus, when a forced induction engine is under load the intake manifold can no longer be used to draw blow-by gasses out of the crankcase and will instead begin to exacerbate the problem by increasing crankcase pressure. It is then the job of the PCV valve to isolate the intake manifold and crankcase when the intake manifold is pressurized and allow the flow of blow-by gases out of the crankcase when the intake manifold is under vacuum. In addition to this added role, in boosted applications cylinder pressures are much higher, and consequently, more blow-by gases are pushed into the crankcase thus making a fully functional PCV system all the more important. Carbon build-up in intake systems Carbon build-up in the intake manifold will occur when blow-by gases are allowed to permanently contaminate the intake air because of a failing PCV system. Carbon build-up or oil sludge from blow-by gases on intake valves are usually not a problem in port injected engines. This is due to the fact that the fuel hits the intake valves on the way to the combustion chamber, allowing the detergents in the fuel to keep them clean. However, carbon build-up on intake valves is a problem for engines with direct injection only, as the fuel is injected directly into the combustion chamber. Because of this, fuel system cleaners or fuel additives added to the tank will not help clean these deposits. Methods for cleaning these deposits include spraying cleaner through the intake or direct media blasting of the intake valves. Alternatives Two-stroke engines which use crankcase compression do not require a crankcase ventilation system, since all of the gases within the crankcase are then fed into the combustion chamber. Many small four-stroke engines such as lawn mower engines and electricity generators simply use a draught tube connected to the intake system. The draught tube routes all blow-by gases back into the intake mixture and is usually located between the air filter