With my '81, every winter it would have an episode of stalling,
coughing, etc, on one of the coldest days.  A bottle of Heet
would cure it.  This was in the first few years when everything
was new.  I always assumed it was condensation plus some quirk
of the gas tank/fuel line design.

We started getting oxygenated gas in the fall of '87 and I
have never had the problem again.  My '87 bottle of Heet is
still under the seat with the jack and tools....

So I'd suspect condensation way before some kind of leak of
wheelwell water into the system.

                                Richard A. Jones
                                Boulder, Colorado
                                '81 Vanagon  Mr Bus