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Fuel does not pass through the intake valves on direct injection engines, so detergent in the fuel cannot clean the valves. Oil from the crankcase does make its way to the intake manifold and intake valves, via the PCV. That is the source of the carbon deposits that build up on them. Walnut blasting is not the only way to clean the intakes, go to the link I posted, watch the video. It's best to clean the valves yearly, so the carbon build up doesn't get severe enough that walnut blasting is necessary. It's even better to install an oil catch can, which prevents oil from getting to the manifold in the first place.
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