It has been recognized that high ratios of EGR are effective for in-cylinder NOx reduction, the mechanism of which has been largely attributed to thermal NOx reduction. However, engine operations also approach zones with higher instabilities when excessive EGR is used, usually accompanied with higher cycle-to-cycle variations. Although appropriate controlling strategies are capable to set up consistent EGR operations, any drifts in engine control will affect the originally optimized EGR when sufficient feedback control is not available. In reality, without sufficient EGR feedback control, the applicable EGR ratios need to be recessed from maximum allowable ratios in considering discrepancies of EGR control and influences of operating condition variations. An experimental investigation is provided in this paper to propose a method to evaluate the influences of EGR on such instabilities, in terms of independent effects of CO2 addition, O2 dilution, and N2 balancing, with simulated EGR. Extensive tests were conducted using a synthesized intake mixture testing facility. This paper is also a continuation of a cycle-to-cycle variation analysis on such operations reported previously by the authors (Zheng and Reader 1995). The analyses are aimed at identifying thresholds of stable operation with excessive high ratios of EGR whilst without subjecting to the consecutive cyclic disturbances associated actual EGR itself.