Sounds like you are describing a PCV valve (a part of the crankcase
ventillation system). In some applications they may provide a convenient
vacuum tap-off for another accessory, but probably not in your case. The
vacuum diagram label inside your engine compartment should show that very
clearly. Most likely someone just installed a 'universal' PCV valve, with an
extra port and did a poor job of capping it. Unmetered air ('vacuum leak')
sucked through there may cause unstable idling, explaining your fluctuating
voltage. Things might have gotten a lot more 'interesting' if something was
flapping around instead of just allowing a steady leak. Did you notice the
MIL (check engine light) turning on? The computer should have detected this
condition.

> The gauge instantly resumed normal operation when I unblocked the port.
> (Blocked with green electrical tape mostly sucked inside the port-clearly
> no professional install)