isthatahemi

So the carbon is in the intake valve shoulder area. Mostly because of a lack of "wash" from fuel. Not sure that any adidtive will make a difference. A solvent based solvent introduced into the intake should work.....

pprior

EXACTLY. Unfortunately that has been very difficult and especially in some german engines an expensive $1200 manual cleaning ever 10-20K miles has been required to avoid extensive carbon buildup.

It's not the fuel injector that gets carbon buildup it's the air intakes, which are pulling crankcase gases in and building up carbon.

It IS a real problem on many models of DI engines, and the specifics of why it happens in some and not others eludes me. It can cause 50+ HP drops over time however.