1993 Lexus ES300 fuel /exhaust system troubleshooting

Daughter's '92 Lexus ES300 has recently begun idyling rough (coughing between approx..75 and 1.2 - 5 rpm). The fuel exhaust fumes are white, emitting a lingering - but not thick - cloud from the pipe with a rich smell of gasoline.
I do not see leaking fuel lines or disconnected vacuum hoses under the hood.
Any ideas where to look for problem?

thx,
Steve