I have LMM with 408,000 miles on it. It is building pressure in the coolant system. It started off gradually and now it is puking coolant out the overflow. I have an EGR blocker plate installed. Could the EGR cooler be leaking exhaust gas back into the cooling system or does the blocker plate stop the exhaust gases reaching the cooler? Im new to the duramax as i bought the truck with 392,000 miles. ANy help would be appreciated. Just hoping its not the head gaskets!!!