Mike, Check the line going from your charcoal cannister to your EVAP valve. There should be a small white filter on the line right before it goes into the valve. The problem is, not all 1998's came with this filter installed from the factory. If you do have it, you probably should replace it. If you dont, go get one and retrofit it onto the line. Then take some carb cleaner and clean out both ports on the evap valve. Hook everything back up, clear your codes, & hopefully youll be good for a while.