that runs from the back of the IP to the intake manifold. The vacuum in this line controls the amount of fuel in relation to throttle position and the vacuum supplied here performs a balancing act with the springs that push on the diaphragm inside the IP. At full throttle there is no vacuum and the governor spring pushes the diaphragm which in turn is attached to the rack and pushes it to full fuel.
If this line gets disconnected or gets a leak in it you will have a runaway with lots of smoke as the rack will go full forward and give you full fuel.

Ths shutoff should still work though... If this line were to break though I have no idea how the engine would get oil to burn from this connection...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique