If you have air in your coolant system, gaps in the coolant if you will,
when that air flows through the IACV, it causes the idle to drop. When I
had my IACV not attached to anything when I had ITBs (Individual
Throttle Bodies) on the Civic, the idle was horrible. We made a plate
to bolt it to that sealed it off properly, and the idle returned to
normal.

When we reinstalled the normal manifold (ITBs aren't too good for daily
driver, I learned this the hard way.) and we had everything back to how
it should be, the idle was still sparatic. After about 10 seconds of
bleeding, the idle returned to normal idle.

Hope that answers your question.

-Wes

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The temp sensors all have to be immersed in coolant or they convey a
wrong reading to the computer. If there's ANY air in the system it will
confuse the computer at some time in the warm-up cycle. Filling the
reservoir is the first place to start after topping up the rad. Air
can't leave if there's no 'spare' coolant to replace it. Fill the
reservoir to MAX before driving, two or three times. Tap water is called
HONDACIDE.

'Curly'