Lots of info by
IHateHakes in this post,
http://www.toyota-supra.info/forums/showthread.php?t=7128&page=17&pp=10
Good luck! your problem could be many things, rad cap, water pump, or even a blocked radiator. The symptoms of a BHG are similar to having worn impellors on a water pump. By the way don’t rely on the temperature gauge to tell you anything, its not until you nearly run out of coolant that it will start to increase.
Is your block tester kit similar to this one?
http://cgi.ebay.co.uk/USA-UNIVERSAL-ENGINE-CYLINDER-HEAD-BLOCK-TESTER-KIT_W0QQitemZ160121367108QQihZ006QQcategoryZ30922Q QtcZphotoQQcmdZViewItem