I had the same problem with my '87 560SL.

It is apparently quite common (as someone already mentioned) for the fuel caps to go bad. Definately change this first (only use an original MB cap.. not aftermarket!)

This made a big difference in my car. However, the fuel smell never totally went away. Not sure if I was smelling residue of fuel in the car or something (I only owned the car for 6 mos after changing the fuel cap). But there was definately a big change in swapping out the fuel cap.

Neal

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'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi) SOLD
'07 S550 4MATIC 235,000km (146k mi) SOLD
'07 GL320 CDI 4MATIC 348,000km (215k mi)
'13 GL350 BlueTec 4MATIC 170,000km (105k mi)
'14 SL550 72,000km (43k mi)