the only way to clean the EGR valves is ro take the valve cover off and clean them manually. carbon deposit on the valves can affect the fuel air mixture by absorbing some of the fuel and causing rough idle. additional buildup can case loss of compression and can lead to engine damage. the valve cover is usually located on the top of the engine usually with rubber breather hoses connecting it to the air intake.
here is a link to a website i'm sure you will find very helpful enjoy:
http://skinnerbox.steaky.org/Service/W124/Index/103index.html