My 97 subaru occasionally idles too fast. I found that this was correct once after buying gas so suspected might have been a loose cap. Tightening the cap does not always help, however. I have cleaned the gasket on the cap and the filler neck with alcohol. This helps, but sometimes for only a day. Is the cap itself bad? Coincidence that it stops usually when I clean the cap? The is a very small amount of non-loose rust in the filler neck also where the gasket meets. It is nearly as smooth as the surrounding metal. Bad gas cap maybe? It is the original.