A method of operating a fuel system to determine the extent of internal wear of a fuel pump thereof comprises driving the fuel pump, moving a metering valve member to a closed position in which the metering valve bears against a resilient stop, increasing the operating speed of the fuel pump to increase the pressure applied to the metering valve member urging the valve member towards the resilient stop to a point beyond which further movement of the metering valve member occurs, sensing when the further movement of the metering valve member occurs, and sensing the operating speed of the pump at the time when the further movement occurs.