Hello All, I just signed up as I am having some issues with my wife's 2008 Cobalt LT. It has 86K Miles, everything runs fine its been overheating lately. It used to be stable at 189 F, but it started goin up to 200's, 210's up till 230 specially when going up hill, weird part if im going down hill it drops 20 degrees within minutes I Investigated and I had low coolant level, filled it up, it kept fluctuating from 189 to 200's this time. but it was running with no overheating issue any more. However a friend said it could be a bad thermostat with fauluty reading and suggested i replace. He swapped it out for me, and while replacing he drained the anti freez (dont know if he is supposed to do that, anyhow, he was able to put in the new thermostat, and filled the car with dex cool halfway and the rest with water as we didnt have much. now the car is worse, its overheating badly it even went up to 250F and kicked in cooling mode. He assumes there is air trapped. How can I purge the air out from the system ? is there a better solution? What wronge did he commit? I appreciate everybody's help and input!