I use a combustible gas detector for finding the source of these sort of problems and I'm usually able to find them. I have even fixed it near various parts and then road tested and it'll beep like crazy as a fuel/fume leak occurs. Has anyone located a fault like this with a smoke leak tester? I've not tried myself. I noticed Snap-On are selling filler cap adapters now for their leak tester. If you could activate the solenoid for the charcoal cannister while the system was under test surely that would be the whole system filled with smoke. The most awkward intermittent smell of fuel I had was on a Merc A class where it was from the plastic fuel rail feeding the injectors.

Liam