The best way is to remove it from the engine and take it apart then soak it with oven cleaner or in a bath of cleaner for a bit. That will soften the gunk up. Then using a long flat flexible blade scrap the gunk out then power wash it or carb/brake cleaner it out. I did this on 3 plenums and it made a big difference in throttle response and smother running. The EGR is simple enough, take it of and run a wire up the nipple and clean it that way. The EGR port runs the lenght on the plenum also, so clean that good when you have the plenum off with a bottle brush and hard wire.
