Each time I have ran a purge the engine is always quieter afterwards. The gunk coming out is black. The last time I did it I forget i had the car idling and came back 25minuets later to a nearly empty can and a soot stain on the drive way. Must have worked well and the engine was of course quieter.
I do like the idea of letting it soak, I figured idle would have a similar effect.

__________________
Nosce Te Ipsum "Know thyself"