EGR (Exhaust Gas Recirculation) is a way of reducing exhaust emissions by decreasing the combustion temperature. It does this by introducing some oxygen-poor exhaust gas into the inlet air stream.
Controlling the engine inlet air temperature is important on turbocharged diesels as it affects exhaust gas temperature (EGT). If the exhaust gas temperature is too high it can cause damage to the exhaust valves, cylinder head, and turbocharger. To prevent the EGR gas from raising the inlet air charge temperature (IACT) too high, the EGR gases pass through an EGR cooler that transfers excess heat from EGR gas to the engine coolant.
Due to the high temperatures involved the EGR cooler can crack and leak coolant into the engine.