For what it's worth, my SL is straight K-jet (no emissions controls) and no cat (Euro model). On startup, you can definitely smell gas when the cold-start injector is open. I run it on the lean side, and when warmed up, you can still faintly smell gas. Still get in the mid-20s on the road though. It could just be that without the cat, you smell the fuel.

I'd be leaning more towards a defective cat than anything else. My dad's 2012 F-150 3.7L V6 had a really irritating rattle. Took it to the stealership and the driver's side cat was damaged (broken away internally). Truck still under warranty, 13K miles on the clock.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)