If the problem is your EGR cooler, you will most likely end up with either coolant in your Turbo exhuast gas cooler return line, and/or coolant dripping out of your tailpipe. When an EGR cooler goes it leaks coolant in the turbo air intake and you can end up with coolant getting into the combustion chamber and blowing a cylinder or a head gasket. Coolant don't compress! Comparitively, there isnt that much coolant running through the EGR cooler as through the rest of the cooling system. If your blowing coolant back through your coolant bottle I would think either your water pump is not pumping coolant or there is a clog in the flow. I don't know if this is an issue with GM, I know its not with Ford, but my wife's BMW's cooling system will not work properly if there is air in the cooling system. The air has to be purged or the car overheats and blows coolant out the coolant bottle, Have you tested your water pump pressure? I think you have more of a actual coolant circulation issue going on rather than an EGR cooler problem. Don't know if this helps but some things to think about.