1998 Dodge Caravan Raw fuel smell

My 1998 dodge grand caravan with 170,000 miles has an intermittent raw fuel smell. This seems to only happen when it is below forty degrees. The colder the temperature, the more often I smell the fuel smell. I notice it mostly when the heater is on with the recirculate set to off. I can smell the raw fuel smell outside the vehicle as well as inside. When the recirculate is set to on I do not get the smell and do not notice it outside the van although it may be present. I have taken it to the shop twice and each time they have not been able to replicate the smell.