Codes read on Expedition

My check engine light is on and I had the codes read. The code that was listed is the DTC PO455, EVAP Emission Control System Leak (Large). I had them read at Advance Auto Parts. The guy didn't really know what the problem was. Does anyone have any suggestions as to what I should do now? Any help is appreciated. It is a 1998 Ford Expedition, Although I am looking at the little paper where the codes are listed and it says Vehicle scanned as a 1999 Expedition. Would that make a difference?
Thanks,
Eddie

My check engine light is on and I had the codes read. The code that was listed is the DTC PO455, EVAP Emission Control System Leak (Large). I had them read at Advance Auto Parts. The guy didn't really know what the problem was. Does anyone have any suggestions as to what I should do now? Any help is appreciated. It is a 1998 Ford Expedition, Although I am looking at the little paper where the codes are listed and it says Vehicle scanned as a 1999 Expedition. Would that make a difference?
Thanks,
Eddie

Eddie,

I have the same problem with my 98 EB. I replaced the gas car (as suggested by a mechanic) and cleared the code. Most of the gross leak Evap codes can be attibuted to the gas cap being bad. If you can get yours tested, I would suggest that. If you cannot, I would start there and replace it first. It should be less than $15.00 to replace from AutoZone, NAPA etc.. If that does not work, you will have to start tracing the evap system and check the components and hose connections. It could very well be a loose hose on the Evap system. If not than one of the Evap valves might be bad. The Haynes manual on this subject does a pretty good job of walking you through the system and finding the valves.
Good luck-
Alex