You'll want to check the fuel pressure regulator and the fuel pressure dampner located on either side of the upper plenum in plain view. If either of those two, or both have a ruptured diaphram they will suck fuel into the intake at idle when vacuum is greatest.
Give either a quick smell test by removing the vacuum hose and smelling for fuel or looking for evidence of fuel. To properly test them you should use a vacuum tester which simply is a hand pump that will pay for itself the first time you use it.