Abstract: The present invention is directed to an on-board diagnostic system for detecting malfunctioning fuel injectors in an internal combustion engine during engine operation. A fuel injector control means is provided for individually actuating fuel injectors operatively connected to a fuel rail. Pressure sensor means are mounted to the fuel rail for sensing transient fuel pressure waves resulting from actuation of the individual fuel injectors and for generating a pressure signal in response thereto. Signal processing means are provided for processing pressure signals from the pressure sensor means and for generating an output signal upon detecting delayed opening or delayed closing of one or more injectors.

Abstract: An on-board diagnostic system is provided for determining the presence of vapor in a fuel supply line of an internal combustion engine during engine operation. Sensing means are operatively mounted to the fuel supply line for sensing transient fuel pressure waves resulting from actuation of one or more fuel injectors. The sensing means generates a pressure signal corresponding to the transient fuel pressure waves at least over a selected test interval. Signal processing means receives the pressure signal and determines its resonance frequency over at least a selected frequency range. The test interval resonance frequency is compared to a stored resonance frequency corresponding to an acceptable vapor level in the fuel supply line. The signal processing means generates an output signal upon detecting a value difference greater than a preselected amount (e.g., greater than zero) between the test interval resonance frequency and the stored resonance frequency.