The IAC valve is noted to be in the "closed" position when unplugged. It is also referred to as a "normally closed" valve.

Question: Is this valve positively closed when unpowered? By that, I mean that NO air may pass the valve. Or is some air allowed to pass the valve in the closed position?

 

There is spring pressure acting on the valve to keep it closed when unpowered, but it's not real strong. So not sure how "positively closed" you mean.

 

how to test it
How to Clean and Test your IAC - Ford Bronco Forum

 

Very good article. I have read it several times over the years, including again this week. The author is great! There is as much or more information in the commentary. Don't fail to read it all.

My '86 is a one owner. I have disassembled and cleaned my truck's original Hitachi IAC many times. Today, when disassembled, I noticed that I could pass quite a bit of air over the valve and its seat. Blowing from the manifold side to the TB side. A quick carb cleaner bath and it had a bit less flow. So I did a bit more aggressive cleaning. By aggressive, I mean that I rotated the valve in the seat, sort of a lapping in with carb cleaner. It nearly sealed. But... When reassembled, the solenoid raised the valve out of its seat just a bit.

My conclusion: The IAC can probably seal if the valve and seat are very clean and undamaged. If an otherwise sealed valve becomes slightly open with the solenoid installed, is this normal? We know that the valve movement occurs in a range between 20% and 80% duty cycle. If the valve is "closed" at 20% then it should still be completely closed at 0%, or no power. The one I have is not closed. Regardless of what we discuss here, I should have a definite answer a few days. I have a new Motorcraft CX-1828 on order.

In the meantime, I may clean up a valve I have from a '88 Crown Vic to see what it does.

 

I wonder if the spring has just gotten weak, being the positive shut-off, for valve.

 

Looking in that direction, I noticed that the pintle in the solenoid is also spring loaded.

And it has enough tension to overcome the tension on the valve pushing against it. So, we may rightly conclude that the valve side spring has become weak. The new valve may confirm this.
I disassembled the Crown Vic valve. It does the same thing.

 

The new Motorcraft IAC also does not positively shut off. It is the same as the valve original to the truck and the Crown Vic valve. The unpowered solenoid lifts the valve piston approximately .005" off its seat.