The evaporative emission control system for an internal combustion engine purges the vapor collection canister to the intake manifold through a purge regulator controlled by the engine ECU. The purge regulator comprises a diaphragm valve and an electronic vacuum regulator. The purge regulator functions to allow a purge flow rate correlated with a control signal from the engine ECU and manifold vacuum, to maintain the purge flow rate substantially constant in response to certain changes in the magnitude of manifold vacuum, and to re-adjust the purge flow rate in correlation with changes in the control signal from the engine ECU. The diaphragm valve comprises a tapered valve element that progressively reduces the flow restriction as the valve element is increasingly opened.