someitmes it means you left the gas cap lid loose last time you filled the tank. the vacuum system trys to pull a small vacuum on the tank to keep the fumes from exiting into the atmosphere. its just a polution control system. disconnect the battery cable for 5 minutes, reinstall, and see if it happens again.--- see if the gas cap lid is good seal.
I redid the gas cap and reset the check engine light with my code reader and so far the cel has stayed off thru about 50 miles of city/highway mix driving. I was worried because i know my fuel lines are starting to pit from rust but not sure if it would cause a vacuum leak. i do know that whenever i go to fill up the tank when i loosen the gas cap it makes a loud shhhhhhhhhhhhhhhhhhh sound from the pressure build up and it does this every time. is this good or bad?