I'm not sure what point you're getting at here?
In port fuel injection the fuel is injected at the base of the intake manifold runners and directly into the port in the cylinder head. Fuel does not travel through the intake manifold.
Yes, but gasoline is washing over the valves on the way to the combustion chamber. This prevents a buildup of carbon and oil on the backs and seats of the valves.
In direct injection there is no washing effect of gasoline over the backs of the valves and carbon buildup can result. In the case of direct injection the oil separator may serve to help keep the backs of the valves cleaner.
“One of the biggest problems with direct injection is that the fuel is no longer being sprayed onto the backside of the intake valves,” Laskowski said.
This mist of gasoline actually helps keep the intake ports clean."
See: Is Carbon Buildup a Problem With Direct-Injection Engines? » AutoGuide.com News
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