The exhaust gas contains water vapor and carbon dioxide which both have lower heat capacity ratio than air. Adding exhaust gas therefore reduces pressure and temperature during the isentropic compression in the cylinder, thereby lowering the adiabatic flame temperature.
In a typical automotive spark-ignited (SI) engine, 5% to 15% of the exhaust gas is routed back to the intake as EGR. The maximum quantity is limited by the need of the mixture to sustain a continuous flame front during the combustion event; excessive EGR in poorly set up applications can cause misfires and partial burns. Although EGR does measurably slow combustion, this can largely be compensated for by advancing spark timing. The impact of EGR on engine efficiency largely depends on the specific engine design, and sometimes leads to a compromise between efficiency and  emissions. In certain types of situations, a properly operating EGR can theoretically increase the efficiency of gasoline engines via several mechanisms:
* Reduced throttle losses. The addition of inert exhaust gas into the intake system means that for a given power output, the throttle plate must be opened further, resulting in increased inlet manifold pressure and reduced throttling losses.
* Reduced heat rejection. Lowered peak combustion temperatures not only reduces  formation, it also reduces the loss of thermal energy to combustion chamber surfaces, leaving more available for conversion to mechanical work during the expansion stroke.
* Reduced chemical dissociation. The lower peak temperatures result in more of the released energy remaining as sensible energy near Top Dead Center (TDC), rather than being bound up (early in the expansion stroke) in the dissociation of combustion products. This effect is minor compared to the first two.
EGR is typically not employed at high loads because it would reduce peak power output. This is because it reduces the intake charge density. EGR is also omitted at idle (low-speed, zero load) because it would cause unstable combustion, resulting in rough idle.
Since the EGR system recirculates a portion of exhaust gases, over time the valve can become clogged with carbon deposits, which will prevent it from operating properly. Clogged EGR valves can sometimes be cleaned, but replacement is necessary if the valve is faulty.