2002 Hyundai Sonata The gas tank will only accept fuel very slowly

2002 Hyundai Sonata 6 cyl Front Wheel Drive Automatic 70,000 miles. The gas tank will only accept fuel very slowly and in one incidence high air pressure was released when filter cap was removed. The local mechanic removed canister closed valve, was told by dealer that was likely problem, still fills very slowly. Any suggestions? Thanks.