Try all of the above suggested, and as GBenz stated, check fuel lines for leaks, around the fuel filter and pumps. Mine were leaking for awhile, but since fuel evaporates so quickly and the setup is located under the black cover (just forward of passenger rear wheel, underneath) I never smelled it.
The copper washers lose their sealing ability over time so if they're leaking, tighten them down, or better yet, replace. Good idea to replace your fuel filter while you're down there.
When was the last time your car passed an emissions test (if your state requires it)?
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi