we swapped an 03 outback engine with a forester engine. The problem we are having is the check engine light for an idle air control valve. We replaced it twice with used ones and are still having a problem

Are you sure the used ones were good? A new one is not all that expensive so I would try one. This might help ---- http://www.obd- codes.com/p0400

Did you use the outback or forester throttle body, intake manifold, etc? Was the plug a direct fit for the IAC?

What's the code?! Also, try spray-cleaning the throttle body fully (hold the stiff butterfly horizontal to get the top and bottom halves of the cylinder bore clean). IAC's don't like gummed up passageways.

upstateonsite, have you considered replacing the idle air control and throttle body with brand new units?

You just swapped out the entire engine, why stop there?

I have a hunch that it might be an electrical voltage design difference from the JDM engine to the outback ECU. I don't know enough but maybe Boston can help or check out some legacy/outback/NASIOC/etc forums for writeups/gurus.

Hmmm...Walth may be right. I'd be sure to match IACV model numbers. IOW use a Forester IACV with the Forester ECU. New ones are too expensive, so get a used one to swap in. But first clean the one you have!