Replaced thermostat (had some junk in it) when car started sudden, rapid overheating....turning heater to full didn't bring it down. Top of radiator had cracked, so replaced that, new cap as well. Started to flush radiator prior to filling with fresh coolant, but car ran up wayyy too hot sitting at idle. Ideas? Is the overheat expected since I hadn't purged system?

 

Typical of an air pocket, look in the FSM for the bleeding procedure and reply back once that's done.