I have a 2003 Civic LX. 1.7L D17A1, manual transmission. I was having engine surging after rebuilding cylinder head, and I was getting error codes for idle air control system malfunction and PCV air leakage. I then replaced the PCV and all vacuum hoses, but the problem persisted. I then replaced the idle air control valve, and now the surging has stopped, but the codes still occur. Now, when I come to a stop and put it into neutral, the engine idles at about 1.5k for a second, then climbs to 2k and drops to 800 RPM where it stays indefinitely. If it is left in neutral for long enough (about one minute) the PCV error appears and IAC system malfunction also sometimes appears. This only seems to happen once it is warmed up. I did the idle relearn procedure and the codes didn't appear until 15 minutes of driving afterwards. I am almost certain that there is no vacuum leak because I replaced all vacuum hoses, PCV valve, all gaskets having to do with air intake, head gasket, and also because turning down the base idle kills the engine, so there is no significant air intake to the engine other than where there is supposed to be.

 

Have you attempted to clear the codes? I didn't see you mention that.

 

seems like an intake gasket is leaking