Onboard Diagnostics for vehicle fuel system

A vehicle fuel system has on-board diagnostics for vapour integrity testing. The vapour integrity test includes evacuating the fuel system; closing the system and allowing pressure to rise due to air ingress and vapour generation (bleedup) ; making a series of pressure measurements at intervals during bleedup to give a series of timed pressure values; and calculating vapour rate from values derived from the pressure values. Vapour integrity may be calculated from the same values representing pressure gradient and the rate of change of pressure gradient or the vapour rate may be used as a correction to vapour integrity derived by other means.