An isolation method and system is described for distinguishing between turbine case cooling (TCC) and high pressure turbine (HPT) performance faults. A trend is observed in gas path parameter data during cruise and a resulting percent .DELTA. signature across the shift in the gas path parameters is assignable to either an HPT or TCC performance fault. During either fault, exhaust gas temperature (EGT) will shift upward. Since take-off EGT margin is calculated from take-off data, the shift, or lack of shift in EGT margin may be used to differentiate between TCC and HPT faults.