My neighbor has a w126 that has been sitting for years. he got it running on an auxiliary tank (because who knows the condition of the car's tank) and once it gets going, it wants to run away until you starve it of fuel. I'm no guru but I've driven and worked on these a bit and can't figure it out.
With it sitting so long, is there something in the pump that could be stuck that might break free with some cleaner? is the Air/fuel control that is operated by boost pressure somehow stuck in the full boost position? Is there a fuel screw i don't know about. I've worked more recently on Bosch VE pumps for my truck so that power screw comes to mind.

What's a likely problem here? Just brainstorming for now. He got this going a few weeks ago so the system has had fresh fuel in it for a while now.

Thanks.

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'82 240D 224K miles manual transmission
mods: wooden 4by4 bumper, EGR delete and older EX manifold without EGR port, glass pack muffler (cheapest replacement muffler), rebuilt bosch injectors with Monark nozzles

working on: aux electric fuel pump, coolant/fuel heat exchanger/filter head, afterglow, low oil pressure buzzer/LED