Complexity of todays vehicles makes it difficult to diagnose without code reader with a laptop with OBD2 codes can be read. My 2004 Ford Explorer Sports Trac had the same issue. Replaced the Fuel Filler Cap which didn't help. Next removed battery cables so computer could reset. So far this worked for me. Only other issue that may have caused this I had to remove charcoal cannister back of fuel tank to install rear shocks. Air might have gotten into the system when hoses were removed for shock installation. This last statement is probably off base. Check for cracked or split hoses.