Abstract: A vehicle fuel system has on-board diagnostics for leak testing with correction for different operating conditions. An electronic control unit (ECU) is arranged to carry out a periodic two stage leak test, when the engine is running. Stage A includes evacuation of the fuel tank, monitoring bleedup and recording the pressure rise dP_A over a predetermined period A following increase of pressure to a predetermined value p2. Stage B includes measuring the amount dP_B by which the pressure in the tank rises above atmospheric due to vapor generation over a predetermined period B following venting and closure fuel system. The ECU calculates a value X representative of leakage from the difference between dP_A and dP_B using a scaling factor K dependent on measured values of operating conditions, which may include fuel level/fuel tank free volume and/or ambient temperature.

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

Abstract: A vehicle fuel system has on-board diagnostics for leak testing with correction for different temperature variations arising in an evacuation phase. Pressure is measured at intervals during the evacuation phase and a correction is made to the loss of vapor integrity indication based on the values and timing of the pressure measurements made during the evacuation phase.

Abstract: A vehicle fuel system has on-board diagnostics for leak testing with correction for different operating conditions. An electronic control unit (ECU) is arranged to carry out a periodic two stage leak test, when the engine is running. Stage A includes evacuation of the fuel tank, monitoring bleedup and recording the pressure rise dP.sub.-- A over a predetermined period A following increase of pressure to a predetermined value p2. Stage B includes measuring the amount dP.sub.-- B by which the pressure in the tank rises above atmospheric due to vapor generation over a predetermined period B following venting and closure fuel system. The ECU calculates a value X representative of leakage from the difference between dP.sub.-- A and dP.sub.-- B using a scaling factor K. The test duration is adjusted in accordance with measured values of fuel level and ambient temperature to ensure that the measured pressure changes are dependent on leakage over a range of operating conditions.