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I'm not sure what else to try. The 1000-1200 RPM idle while coating doesn't seem excessively high, but the engine isn't particularly happy idling at that RPM and it feels quite rough and annoying. I can live with that though. Now the problem is that if I turn the AC on the idle increases to around 1600 RPM and the engine smooths out, but the unnecessarily high idle caused by running the AC reduces my coasting MPG significantly. I wish it would just idle at ~800 RPM all the time like when I'm stopped.
I thought of a couple things I overlooked when I checked for vacuum leaks- the brake booster and PCV valve. The brake booster will hold its vacuum assist for hours after the engine is shut down, so it seems alright. As for the PCV valve, I tried disconnecting it and plugging the vacuum line to eliminate it as a possible problem and that made no difference.
About the only other thing I can think of is to find a way to use the stock D17 IAC instead of the slightly larger one on my K20Z1 RSX-S throttle body, but I don't think that will make any difference since the ECU seems to be in control of the idle speed. I wonder which other inputs it uses to determine idle speed.
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