Location: Carson City, NV Class Champion Reno SCCA 2018
Carbon on GDI intake valves?
As a retired guy with arguably too much time on my hands, I was recently in touch with this company in regard to their intake valve cleaning product for gasoline, direct-injection engines. How does a product injected into the combustion chamber end up on intake valves, and how do intake valves in a GDI engine get dirty anyway?
With respect to your question on valve deposits and detergents, that question could be answered by citing the phenomenon that, despite the fact that the fuel is directly injected into the cylinder as you note, deposits still build up on the underside of the valves.
Here's a reasonably good explanation of how they buildup and how they would be removed by fuel detergents.