Abstract: A diagnostic envelope is used to analyze measurements from an internal combustion engine. Measurements may be from a partially assembled engine during a cold test to detect subtle mechanical anomalies, or from a fully assembled engine during a cold test or a hot test (with active fuel and spark). The measurements represent vibrational amplitudes corresponding to various crankshaft angular positions of the internal combustion engine. The first step in generating the diagnostic envelope includes filtering the measurements to enhance local variations. Filtered measurements representing a number of engine cycles are then combined to reduce random cycle-to-cycle variations. The diagnostic envelope is then generated by subtracting corresponding measurements obtained from engines with known operating characteristics. The diagnostic envelope is used in analyzing and/or classifying the engine.