Are you sure the 2 hoses on the AUX Air Valve are firmly seated. If they are the original hoses, that's 28 years, they could be lose or cracked.

Have you cleaned the valve, if you take it off the car and look thru it you can see the slider, spray with carb or brake cleaner, slosh it around inside a bit and drain.

You should be happy that you have this system instead of the newer one. This is a simple mechanical valve that is open when it's cold, and gets closed when warm (heated by the electrical conection and the heat of the engine, and helped along in closing by the virbration of the running engine). The electrical connection is just for a heater element, that's all it does. It's basically an automatic choke.

The newer systems are always 'vibrating" electrically. Plus they cost many times more to replace.

I've only had one of these valves go bad and the problem was that it would no longer close enough when warm, so the idle was always high. The other one I've got is the original on a 1980 car.