I gather that you bought the SL500 privately without having a pre-purchase inspection performed by a MB-trained tech. If that's the case then I suggest taking the car to a local independent mechanic who specializes in MB and has a good reputation for honest, reasonably priced work; and have him diagnose the idling problem and inspect the entire car for existing problems. You failed to mention the mileage on your SL500.

On the other hand if you bought the car from a dealer go back to the Service Department ... you must have some sort of warranty.

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Fred Hoelzle