2000 i30t overheating/no heat issue
 

My mechanic recently replaced the radiator to fix the issue, but it still continues. When idling in a drive-thru or such, the temp gauge will start climbing. Starting to drive at normal speeds will cause it to drop back down. At the same time, I have no heat unless the car is moving at a decent pace(40mph+). I suspect the thermostat, but thought that it would only cause one or the other. Not both issues. Any help would be appreciated. Thanks in advance!

First, confirm that your fans are working. Both fans will kick on when the coolant temp reaches 203 degrees. They will cycle off after about 15 seconds when the temp gets below 203 degrees.

Secondly, park on a steep incline, remove radiator cap, and burp the system of any air. Make sure your heater is on too, and on full blast on high temp heat. Rev the engine to 1500rpms for a few minutes and see if you see air bubbles in the radiator.

Third, feel the upper and lower radiator hoses once the car is nice and warm. If only the upper is hot, then either you have a bad thermostat and/or a bad radiator cap.

If possible, go back to the mechanic and have him pressure test the radiator cap and cooling system. It only takes a few minutes. If the mechanic installed a new radiator to solve your overheating issue, then he's responsible for not fixing/diagnosing things correctly.

I'd def say it's air in the system.

Just park it with rad cap off, and rev it out.