20v Silvertop Vacuum activated switch identification Q

[dank]

Got the 20v running smoothly in the sierra now using Stock A/T Ecu with a bypass on the transmission "in gear" signal.

Sounds like it idles fine, revs nice and smoothly but "just" off idle into acceleration its seems to hesitate a bit and then picks up very nicely.

We redid the starter and fuel pump relays just for ease and dropping out a heap of wiring we didn't need. The image above shows what looks like a vac operated valve/switch that isn't currently connected. All other vac lines are connected up.

Can anyone identify this for its function, it's ecu pinout code to connect the wiring to if needed and whether we need to run it at all?

We had a look through about 4 different pinout codes and two different wiring diagrams we had but couldn't locate it.

[IAN20V]

looks like the aircon idle up valve