If your 300SD is like my 85 300D, there is no throttle cable. Instead, the pedal rotates a rod in the engine bay, which controls the fuel injection pump (diesels have no throttle). Where the rod bolts to the firewall, there is a rubber isolator. It tends to split and new ones cost >$50. I just tied across mine with several wraps of stainless wire. Have someone press the pedal while you watch under the hood. I bet you will see the rubber open up.