2000 Impala fuel issue

My mother's 3.8 Impala will not accept fuel unless it is trickled in slowly. First thought was maybe something in the filler neck, but that doesn't seem
likely. It's been cold out, so I haven't taken the neck off yet. Anyone have any ideas on what to look for? It takes 10 minutes to fill it with a 5 gallon gas can. I have a day off on saturday so I'm going to take it apart then. So, any advice would be appreciated.
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