Perfectly explained and there's a few hours work in sorting that out. You have to wonder what design engineers were thinking ??? On the whole is the 2.2 a good engine ?

The carbon buildup has to go somewhere during your hydrogen cleaning. You can also run the risk of the carbon getting trapped between the valve and valve seat during your cleaning efforts. Worse, you can get carbon trapped in the rings possibly causing rings to break. Best case is to mitigate EGR using a catch-can.

I've seen before and afters of people who carbon clean an engine with hydrogen and it seems to be a gimmick. Every before and after I see, the piston and valves look slightly cleaner, but still absolutely caked in carbon. Seems to be a waste of money, taking the intake manifold off and cleaning the valves by hand or with walnut blasting is the best way to clean them.