EGR stands for Exhaust Gas Re-circulation and EGR system is the system that is used to reduce the emission of NOx.
NOx is produced by the combustion of gases.
EGR system does not have much effect on the performance of the engine. It helps in reducing the emission of NOx by adding a little amount of exhaust gas to seep out.
But this leaking EGR may cause problems in driving such as detonation, the problem in starting the engine. This leaking exhaust gas is enough to get diluted into the air or fuel to get the cooling effect from the temperature of combustion.
This helps in maintaining the combustion temperature under 1500 degrees C so that nitrogen and oxygen do not react and form NOx.