hey all, just an addendum to many who ask why your Z may overheat. as stated by 'Sblake' and others. try changing your thermostat to a NISSAN one. get rid of the aftermarket one. in the pic, the NISSAN one is on the left and as you can see the water flow is greater through the larger diameter NISSAN stat. good luck.

Also there is supposed to be a little hole in the flat disk part. This hole is to go on the upper side when it is set in place. This will allow the air to pass though and not to collect and keep the water from opening the stat. Especially when first starting after a total coolant drain.