The system is pressurized...if your cap leaks, then you will have problems with gas spewing out of the cap (especially on hot days)!
But I am concerned about the amount you are losing! A cap leak would lose trace amounts...perhaps you have a leak elsewhere.
It doesn't take much fuel to create a lot of odor!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car