I am inclined to recommend taking it to an independent shop that is up to speed on diagnosing. I may be able to locate one in your area, or help you to. A shop is only as good as the staff-that includes dealers. There are some good bad and ugly in all of the industry.
If you replaced the gas cap, and put the tank at 3/4 full. Then drive normally for a few days, that MAY solve it. This isn't a problem that is going to need immediate attention.
What will be tough for you to go after it yourself it may be the tools of the trade, such as possibly a smoke machine ( a machine that injects smoke into the evap system and exposes leaks, or if there is a faulty computer ( which may come down to a guess) or a scanner that can operate the system. NOt to mention the knowlege of the system.
I'll help where I can, as you like. Ask away and I'll try :)
Thursday, March 29th, 2007 AT 5:56 PM