backfiring through engine

:banghead: 2000 5.4L Expedition is backfiring through the engine and dying after a couple seconds of being started. PCV valve seems to be loose, but I checked the gromit and it seemed OK. I think its just an effect of the backfiring because I held it down tight when starting the car and it didnt make a difference. The guys at the auto store though it might be a vacum leak, but all hoses seem to in attached. Any ideas where I should start looking or how to begin troubleshooting?
BTW - it was running fine yesterday, and today when I tried to start it, it began violently backfiring in the engine.