EGR is designed for one purpose, to reduce the amount of NoX or Oxides of Nitrogen produced by the combustion process.
Two main factors contribute to NoX generation.
Temperature and Pressure
By passing burnt exhaust gas back into the intake system, we reduce the amount of fresh air available for the fuel to mix and burn with. This reduces how much temperature is generated through each combustion cycle.
By rights, this does not pose any kind of problem apart from reduced torque. So by removing this system, you generally will notice a small improvement in economy and power. But, the EGR system is only operating at idle and light cruise conditions. So high load engine running is completely unaffected.
The problem arises from the engine breather system which is recirculating blow by or oil vapor back into the intake system also.
Once these two mix (Exhaust gas and Oil) it forms a horrible paste that resembles Vegemite and builds up on the internal walls of the intake manifold and head ports.
Once there is a coating on the manifold, it provides a nice sticky surface for more EGR build to stick to also. Snowball effect.
The more buildup that is stuck to the manifold and head ports the smaller the ports become.
Smaller ports do not flow as much air as larger ports obviously.
Over time, this significantly reduces the amount of airflow through the engine. The boost level will remain the same however (remember Pressure and Flow are two different characteristics of an engine)
On All older diesel engines without electronic control, the injector pump will continue to inject the same quantity of fuel regardless. It has no idea the engine is down on airflow.
On most newer DI/CRD engines, the Air flow meter is referenced but this does not always mean the fuel system will deliver less fuel with the reduced flow. Often it is simply throttle position and boost that is used to determine how much fuel is being injected.
With less oxygen in the cylinders, the same amount of fuel will not burn as efficiently and it will continue to burn for longer through the power stroke. Significantly increasing EGT, and lowering the potential efficiency and torque output of the engine.
There are two way’s to avoid dilemma
1) Remove the EGR system early on in the picture so it cant cause these problems.
2) Use a good quality engine de carboniser at regular periods (every service).
Some cars are seriously effected by the removal or blanking of the EGR system.
With the advent of ECU remapping, it has become far more desirable to actually turn the system and any associated code off rather than just block it mechanically.
This has the effect of the vehicle appearing standard if it being looked over by the authorities.