It looks like a lot of P0505 errors are a result of a bad IACV screwing up the ECU. That is not my problem. There was nothing particularly wrong with my OEM IACV, but I am chasing down a LTFT of 10% that goes away at higher RPM (usually this means small vacuum leak). I took the IACV off of the throttle body and the gasket between the two was a cracked and flattened mess. So, I put in a replacement (Hitachi) IACV that cane with a new gasket.

Now I have a P0505 error that wasn't there before. I suppose the ECU is rather particular about the IACV and customizes itself to the IACV characteristics. I tried to conduct the Idle air re-learn procedure in the FSM, but that didn't seem to do anything. I've driven the car maybe 100 miles and 5 or 6 warm up cycles since installing the IACV. How long does it take the ECU to get used to the new part? And has anyone else had luck getting the ECU to relearn the idle air parameters?