Is the throttle linkage coming all the way back to the idle stop on the back of the IP?
There is also the possibility the engine is getting fuel from some place else, like engine oil getting into the intake at the turbo charger. Almost a minor run away perhaps.
Check the ALD to make sure it has not failed and pushing for maximum fuel.
I don't know if it can fail in a way to do this. Not that familiar with a 79SD. Just a thought.

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1984 300SD turbo 126
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