There is a fan. Look here : http://www.shio.org/diary/981230/I don't know if that helps... You should take off the cover and check it out. Either add or replace a fan if it's broken.
The original iMacs, with the tray-loading CD, have a fan, however later slot-loading CD G3 iMacs do not. Besides the fan issue you should also make sure that there is no dust build-up in the case and that there is room around the iMac for air to circulate.
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