Pontiac Problems? Ask a Mechanic for Answers ASAP
Remove the thermostat and see if it still overheats without one in. If not, get a new thermostat, obviously that one is bad. If it still overheats, check the flow through the radiator, perhaps flushing knocked some stuff loose and is clogging it up now. You could check that first before removing the thermostat, start the engine allow it to warm up and shut off the engine. Then feel the entire surface of the radiator core, if you feel and spots that are cooler than the rest than you have clogs.
It could be. If there is no air bleeder, pull the front tire up on a curb on the side where the radiator cap is or jack up that side until the radiator cap is higher then the rest of the cooling system, which includes the heater core. Remove the cap, top off the coolant to within 1 inch from full, start the engine and allow the engine to warm up or thermostat open. Then top off the coolant again if needed. Shut off the engine, then grab the top radiator hose in both hands and squeeze real hard with both hands a few times as hard as you can and push any air out the radiator cap hole. Top off the coolant, put the cap back on, lower the vehicle and try that. If it still overheats, then something else is wrong or there is still air in it. Be sure to put the heater on hot, too when doing this but you can leave the blower fan off, so you circulate the coolant through the core.
One good way to tell if there is air in the system, is once the engine warms up, shut off the engine and a glove on try to squeeze the top radiator hose closed. It should be hard as a rock. If you can squeeze it closed there is air in it. If you can even squeeze it a little there may be air in it.
Try that and let me know.
Did that get it bled?