Have the same problem. Most noticable at stop lights when sitting for more than a few seconds. Started freaking out and took car to mechanic, (Enrique of Mr. MB Motors in Tarzana, CA). He checked the usual suspects and traced the smell to the EHA. More info on the thread below. He said it was not a big problem and since the exhaust manifold if on the other side of the engine, was not a safety problem. He said the part is expensive to replace (~$220) and that it really didn't need to be replaced. The only drawback is the "odor" you smell. I'm going to replace it anyway because my wife really doesn't like the smell. As with you, my M103 engined '89 300CE has between 70k and 80k miles.
Gas smell, where to start looking?