Seems like, if you can start it without the runaway condition going into effect, that you could start it, get out of the car and operate the throttle from the linkage, you could see if its binding anywhere, and also be closer to the stop lever. If it isnt the linkage you would think its some kind of ip issue. lets hope its the linkage.

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1996 E300 110,000
1985 300TD 212,000
1980 300D 238,000