I have the upper intake off. What's the vacuum actuator actually controlling inside the intake? I have the aluminum cover off the manifold but the rod spins freely and does nothing. How does this design work?

 

Welcome to the forum

VIS(variable intake system) was only found on the 1997/8 4.0l SOHC, first 2 years of this engine, Ford has used similar on other engines

Purpose of intake manifold runner control(IMRC) is to take advantage of the "rebound effect" of pressure waves that form in the intake runners(tubes).

When an intake valve closes the air that was rushing in is stopped suddenly and a pressure wave forms that heads back up the runner, at the other end of the runner it is reflected back(rebounds) towards the intake valve end again, if this pressure wave reaches the intake valve just as it opens again, its like a mini-turbo charger in that it forces more air/fuel mix into that cylinder
For years and years intakes have been designed to take advantage of this "effect", the length of the intake runner determines at what RPM you would get the benefit of this effect, and its only in a small RPM range for the best benefit

IMRC, in essence, adjusts the length of the runner, so can move the rebound effect within the RPM band

I have never looked into the 4.0l SOHC VIS system so can't say what "normal" would be

Some info here on it: https://www.explorerforum.com/forums...ersion.350782/

 

I'm just replacing the gaskets but my curiosity got the best of me. After removing the iac and the cover it revealed a small buttery fly valve but theres lots of clearance when closed.
So the valve needs to be installed open and then it will close at WOT, correct?

 

Don't know, it doesn't technically "close" or engine would stall, its not like a throttle plate

 

The valve is more a diverter than a gate/flapper. Airflow is routed "short" or "long". The intake is tuned to 2 notes based on rpm.
The '91 Taurus 3.0l V6 had "paralleled" intake routes. One was longer than the other.