Abstract: An engine diagnostic apparatus includes a first transducer positioned in the oil filler neck of the engine for translating the crankcase pressure waveform into a corresponding electrical waveform having a frequency component proportional to engine speed. A second transducer is positioned to translate an exhaust pressure waveform emitted by the engine to a corresponding electrical waveform having a frequency component representative of an engine fault at a frequency proportional to engine speed. The two frequency components are compared to provide diagnostic information.