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I haven't traded my old Civic on a new one yet, so certainly not a road warrior, but if you're referring to carbon build up on the intake valves, here's what we know:

1. Pressurized engines (turbo and S/C) have more piston blow-by than N/A.

2. On a direct-injected engine, no fuel charge ever touches the intake manifold or valves.

The rest is simple deduction: When the crankcase pressure is vented off to the intake, the hot gasses will contain oil mist, which is deposited inside the intake manifold and on the back side of the valves. With no incoming fuel charge to dilute or wash this oil off the hot valves, what else can it do but build up and eventually throw a code?

Solution: Install a fully-baffled oil catch can between the crankcase and the intake manifold.
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