Finished rebuilding my 93 lt1, got it running and everything was going good until about 5 min into it running and the temp began to climb. i turned it off before it got too hot but discovered that the steam return line was the only hot line. this problem was due to the radiator being blocked so i replaced the raidator. started the car and again in about 5 miinutes the temp started to climb so i vented the lines until they were at a solid stream. shut the car off and checked all the hoses this time none of them were hot. i checked the radiator before installing it and it was good. the thermostat works it opens at 180 like its supposed to. i have pacesetter headers not armour coated or wrapped at all so what i think is happening is the headers are heating the temp sensor and causing it to show a false temp because it will show that it cooled down to about 120 after i had shut it off when it showed above 200. just want to know if anyone else has had this problem or knows why this problem may be occuring. thank you all for your help.

 

Possible air in the system. Fill and bleed procedure. Of course, the temp is going to climb some when you are sitting still and the fans are not on.

 

^^ what he said.. been there. my motor is a '94 it had a bleed scew