Idle Air Control

Like most port injection systems, I would guess... right next to the throttle plate. Idle air on such routes a controlled flow past the butterfly.

Never had a crown vic, but on both the Taurus and Explorer it's 'high up' and rearwards on the engine. Very very easy to get to and replace, definatley within the average owners ability to replace. Two bolts, a gasket and a single electrical connector. After replacing drive for 15 minutes or so in all gears and speeds to retrain the computer.

It's cylindrical and about 4 inches in length on the Ford's I've owned. Replacing the IAC valve in the Explorer fixed my issue. Hope you have similar results. This link shows you what to look for (all ford IAC's I've seen look the same)

JP

You will find it below and behind the throttle body at the back of the intake manifold. Removing the TB makes it much easier to get at. While you are there, be sure to clean the EGR ports in the manifold while you have the TB off. They tend to clog easily on that vintage.

Lugnut

I just tested one of these on a 97 ford, You can check for voltage to the unit and check for resistance through the unit. It tested good though.