Theres a leak, check for fuel line cracks or loose connections(engine bay area). Check fuel tank area for leaks also on the fuel lines, fuel pump cover(i think ur's the plastic tank type like the e36). The cap has a rubber gasket.
By the way it the smell strong after filling up to full tank of fuel. If not, like what u say it smells after ur car stops, this is probably fuel line leak in the engine bay. Take a look inside the engine bay after u off the engine.Good luck