Originally posted by WhiteRice
I've heard of this causing the temp gauge to rise, but not mess w/the idle.
But, if it does, the cure would be to take off the radiator cap (when the car is cold), and let it run for about 10-15 mins. Make sure you're on a flat area as well!!! 
heres a funny story.
i changed my girls rad in our old 90 hatchback last year when it was still her daily.
the gd thing kept overheating though, couldnt figure out why...
drive it around the block, it would get blazing hot, bring it home, drive on ramps, check the coolant...coolants fine.
take it off the ramps, drive it around....
put it on the ramps, take it off :doh:
fill the car while on level surface? problem solved :doh: :doh: