I had this exact problem on a customers 87 420SEL. On his, it turned out to be the throttle plate was sticking in its bore. It is a round plate in a round bore and is adjusted at the factory to close completely without binding. As everything wears and gets dirty, it will stick. The linkage has one rod (I think it is at the back of the engine) that has a spring in it so if something hangs it wont bend anything. This rod is only a safety device and I only mention it so you and understand how the peddle can go down but the throttle plate not open. If you follow the linkage to the front of the engine and see where it goes down , you will see where this throttle plate is located. With the aircleaner removed, carefully press down on the big round air valve. Look down and torward the front of the engine and you should be able to see the throttle valve. Spray this plate with carb cleaner, stoping every so often and start the engine so it won't get flooded. If this helps temporarly but starts hanging again, then the throttle valve will have to be removed and adjusted. This requires removal of the fuel dist assembly.
84 500 SEL (307,xxx miles)