Hello. my vehicle is a 2000 Honda civic with 253K miles on it(standard transmission). We just purchased it, and the check engine light came on accompanied by idle surges while stopped. It only happens when the vehicle is warmed up. Curb idle seems a bit high at 1000 rpm. We replaced the IAC(and checked the power to it) with no change. Code is P0505. We attempted to purge all possible air from the cooling system by running the engine at 2500 rpm with the radiator cap off for a lengthy time. The car is not overheating at all. Any help would be appreciated, this is my first post here, and I apologize if this is a redundant question that's been answered before. Thanks.