Engine overheating mystery

[b5a4]

Okay I have an interesting problem, so My car started over heating the other day, and when checked out the motor the top radiator pipe was hot while the bottom lower pipe was cold, pointing to a bad thermastat. So we took the old one out, cut out the guts so It would be open and installed it. Motor ran fine without any issues for last two days. So finally got my new thermastat installed it and it started over heating again.... Checked hoses again and bottom one was cold and top one was hot and sucked in like there is a vaccum on the hose. It seems like the thermastat isnt opening like it should, yet is brand new. Does anyone have any ideas as to why its doing this? Thanks...

[anothernord]

Air bubbles in the coolant?

[b5a4]

no didnt see any, headgaskets have been replaced also

[green97gt]

restricted rad?

[decke48]

does the hose suck in when running or off? usally when hoses suck its caused by a bad radiator cap. and will create a vacuum in the system when the coolant cools off. and if theres a vaccum in the system it lowers boiling temp. (water boils at -30ish psi at avg room temp)

[jey]

Is it an OEM thermostat?

[b5a4]

It was a stant thermostat, I tore it out and boiled it and it appears that its opening crooked and causing it to not open whole way, so Im taking it back and trying again with a new one