Very Bad Idle (Some Times)
 

I have a rare (First 100 built) 1990 300SL with 80,000 miles (The First Year of the 90's SL Body Style Change) with an inline 6 cyl. 3.0 engine rated at 235hp.
It seems either after I refill the gas tank I notice on removal of the gas cap I feel and hear a rush of air exiting from the filler neck. I know that this is a pressurized fuel system, and this is supposed to happen. I have replaced this cap hoping this would solve my on going problem. Now in cold weather on starting the car I hear this noise (Whining under the hood) for a few moments, I assume that this is the air pump which pressurizes the fuel tank. When I hear this I know the car will run good that day, it runs good to very good on those days. Now when the ambient air temp is 50 or higher the car runs very rough on start, and at times very hard even to drive slowly down the street, sometimes even the car stalls. Eventually when on the highway, the car runs good. Now after on the exit of the highway the car is still missing and idling very rough. I have replaced all 6 injectors / Fuel Filter and Plugs. I'm leaning toward replacing the air pump now, but seems maybe the control system that sends the signal to the air pump is the problem. Does anyone know or have had this sort of problem.

Thanks and hope to hear of some other options before I opt to just take it back for the Benz service, which will be Iam sure a small fortune.

Before you spent more money, test coolant temperature sensor.
Also take off air filter and check for broken wires. German vehicles from the 80's have a very bad wires. With age PVC jacket became very stiff, breaking easily, exposing copper to air, then aging process in copper takes place, becomes brittle and corrodes. Keep this in mind when you work under the hood. Check first for the most obvious.