I had a similar problem. Every time I tried to add fluid it would back up & overflow or come out at the bottom of the filler neck. I discovered a mesh filter that hangs down in the tank. I think this is part of the problem. It slows down the flow of fluid into the tank causing it to overflow. If you remove the filler neck you can just barely see the top of it. It looks sort of like an O ring from above. Reach down in there and it will pull straight out. Put the neck back on and try filling. I really don't know how much good this thing does, other than cause problems. That cured my problem on an 94 850, hope it helps you, Jim.