On my '84 CD, the thin black line goes from the Vacuum Control Valve to a vent in the passenger compartment. Fine as is, or simply connect to side of VCV via a piece of tubing. The black box on top of the cam (valve) cover is a 3/2 valve, part of the Exhaust Gas Recirculation system. If completely bypassed, you'll get better performance. The plugged up line used to "tee" into the main line and to the green check valve on top of the VCV. Said VCV is not in either pic. Above info is gathered from a newer model. Your Bowden cable (the black cable ending in a white adjustment on the mount extending from cam cover to driver side, closest to front of car) is set up different. Turbo? Hope this helps.

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83 SD

84 CD