Look for fuel line leaks at the fuel filter-pumps assembly under the rear of the car while it is running. If the fuel filter was changed without using new copper gaskets a leak can develop with sustained vibration. If the leak is minor, it will not affect engine performance but will smell.

Fuel cap is a good bet if no leaks as is a leak at the filler pipe between the tank and the fill hole.

The 560SL also has a history of developing leaks at the fuel distributor but that often is felt in engine performance.