My wife and I have purchased a 1978 300D. I just changed out all filters and fluids and was picking over the engine compartment trying to familiarize myself with the W123 when I saw an apparently disconnected vacuum line. It is on the top of the injection pump. Am I correct in thinking it may be why the idle adjustment knob on the dash does not work? Thanks in advance.

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That spot on the Injection Pump with no line is normal. That is just the altitude capsule and needs no line attached. It has nothing to do with the idle control knob which is driven by a cable attached to the linkage from that knob.

Nice looking 300D by the way! Keep it in nice shape!

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Wow, thanks for the quick response! It appeared to be a static vent. Thanks for the compliment. We bought it from a retired Psychologist that had purchased it new. It is completely stock and has all the manuals and tools, even the first aid kit is compete. I just went out and studied the throttle mechanism and will be ordering new ball links and bushings for it. It appears to be pretty sloppy.

I do believe that is an open vent on your (non-turbo) engine.