[mwebb]

EGR is INert gas - it displaces air fuel mixture

cooler EGR is thought to be better than hot EGR which is why many systems use EGR coolers , some systems that vary valve timing and overlap to get the EGR can not use EGR coolers

although incoming EGR gases are hotter than ambient air
even if fed through an EGR cooler
, when EGR is active
COMBUSTION TEMPERATURE always DECREASES relative to when EGR is not active

when EGR is active , engine displacement /engine size is reduced by the amount of EGR flow .
so a 2 liter TDi becomes a 1 liter TDi with 50% EGR flow
less so with gasoline powered systems which will not tolerate any where near 50% EGR flow

EGR when active
reduces the amount of air fuel mixture in the combustion chamber , creating a cushion and thereby reducing effective engine compression and reducing combustion pressure
which reduces combustion temperature
which reduces NOX emissions , which is a very good thing
which is the primary reason EGR was designed and implemented

EGR is inert gas , it does not change mixture in any way and does not and can not make the system lean or rich IF operating as designed

on a speed density system , systems that use MAP sensor s
EGR problems can change fuel trim significantly

EGR when active displaces incoming air fuel mixture
so less fuel is used , the engine produces less power for the same throttle opening so
the driver opens the air plate to get the same power , which improves Volumetric efficiency , slightly

thus improving fuel economy a wee but measurable amount
EGR is good