The Rochester Quadrajet on this car works, essentially, like all the
other Rochester Quadrajets - venturis, float bowls, metering rods, idle
circuits - same old stuff. The mixture control solenoid and throttle
position sensor are different, though.
Is the heater/AC controlled anywhere by vacuum devices? The brake servo?
Auto box? Remove all these feeds and blank off the outlets. But don't
forget the engine crankcase breather system too when checking for leaks.
Yes, there are systems that run on vacuum signals. The cruise control
is vacuum actuated, and the heater controls send a vacuum to motors
that open or close doors to direct heated air to foot-level vents or
face-level vents in the passenger compartment.
I was getting an erroneous vacuum reading, because I had connected my
vacuum gauge downstream of a check valve that sent vacuum to a storage
tank that was part of the heater control system and cruise control.
Even with the engine off, vacuum was present. That would have tipped me
off right away, but I had disconnected my gauge with the engine running
and did not note that effect. With the gauge connected upstream of the
check valve (that is, between the intake manifold and valve), I now get
a reading that better reflects the actual state of the vacuum. The
vacuum is still at a good level, about 20", but the needle is twitching
at about 10 Hertz, which is (incidentally?) the rate at which the
mixture control solenoid is refreshed.
Right now, I have the gauge on a long enough hose that I can sit in the
driver's seat and watch it. The cruise control and heater controls are
disconnected. Since this did not alter the stalling at idle, those
circuits can be eliminated as a cause.