for serious carbon buildup in the EGR i use oven cleaner, like easy off or something like that. drives my wife nuts cause she never has any oven cleaner under the sink. for the egr, youll want to clean the valve itself, mounted to the head behind the intake manifold, the tube that connects it to the upper plenum, take off the upper plenum itself and clean that passage with a bottle brush, and as much as you can get from the head itself.

i had the same thing happen to me once, EGR vacuum hose popped off, causing spark knock much like you are describing. my two cents