i've seen it , it usually blows out the cap or leaks out through the egr cooler, there's a tsb, but i can't access it right now , on how to test for the leak, lift just the back end of the truck up, remove the egr valve , put pressure to the cooling system, if you see coolant pooling in the egr passages it's the cooler leaking...... if it isn't leking there it's probably head gaskets