2001 Civic LX. Long short story, belt broke leaving me stranded ended up with a shot engine, changed it with one that came out of a 2002, New exhaust manifold converter. Engine runs but noticed immediately it was overheating and a wicked idle surge that wont end. Doesn't seem to be overheating any longer but the surge is there. I've checked connections, burped the coolant but haven't been able to find the problem. P0505 is the code that comes up. Wondering if it could the fuel filter,don't see one on the firewall so assuming it's in the tank.. Running out of ideas.. help!? Not a VTEC

 

Surging idle could be a vacuum leak or a dirty/bad IACV