The electronic throttle assembly may suffer an electrical failure. It is often cheaper to send the valve to a service center for repair that to purchase a new part.Google+
Car Problem Reports
Mercedes-Benz 500SL Throttle Valves May Experience Electrical Failure
Mercedes-Benz 500SL Problem
Visitor, 1992 Mercedes-Benz 500SL, 118,000 mi
Changed plug wires distributor caps , rotors, and now eng will not advance timing and has a miss at idel will not excellarate under load it has not ben fixed to date I think the ECM has gone bad every thing else checks out ok and no code,s are given by the compotor unles you force them to report Joe pelkey.