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A case study involving robustness of engine stability is carried out to test the applicability of some new tools intended for robstness analysis. the system conists of a spark ignition engine and controller operating at idle speed. The problem of robust stability is posed in terms of a polynomial whose root space is required to be in the open left half plane for the system to be robustly stable. It is shown that different assumptions about which intermediate values of the uncertain parameters are physically feasible lead to different root spaces of the polynomial. Two "intermediate value models" both imply robust stability. As a confirmation of these results, brute force gridding of the parameter set is carried out.