One technique used to reduce fuel consumption & pollution is called exhaust gas recirculation or EGR. Some exhaust gas is passed back into the intake manifold of an engine. Sometimes a cooling system based heat exchanger is built into the EGR to reduce the exhaust gas temperature so intake valves won’t be burnt causing failure. Sometimes the heat exchanger leaks causing the engine to run rough & overheat, showing symptoms of a blown head gasket or worse still, a cracked head. When diagnosing overheating & rough running of a late model engine it’s imperative that the EGR is checked otherwise you might spend thousands on cylinder head repairs only to find that the same issue still exsists. By sharing this info hopefully no-one will have to learn the hard way.