New water pump' thermostat,purge line. not heads or gaskets and still running hot

If it was running hot before you replaced the thermostat and the new stat didn't help it, most likely you have another problem. First make sure you installed the new thermostat correctly and not upside down and make sure you have purged all the air out of the cooling system. Your radiator could be clogged and restricted, not allowing coolant to circulate. Your fan clutch could be faulty or the radiator fan is not working properly. You could have a bad head or head gasket causing combustion gases to enter the cooling system. The water pump impeller shaft could have broke---These are the most common causes for an overheating problem. Good luck, i hope this helps.

are you sure that the radiator is not clog ,also check the cap for broken gasket also check the hoses for stiffness i would also wanna run it without thermostat to see what happens ,some times they come defected from manufacturer good luck

Remove thermostat and run the vehicle. do your hoses get tight now? If so, your new thermostat was not opening up. test it by putting it into a pot of water and bring up the temp on your stovetop. if the water boils, and the thermostat isnt open, then get a refund and a new thermostat. Be sure to point the spring end of the thermostat into the engine block when replacing. If your thermostat did open, then your water pump may be shot.

It is probably a head gasket issue if you have replaced the thermo and the pump and it still overheats. You probably can drive it 15 minutes or so and then the gauge starts soaring towards overheat shortly there after. Under completely normal driving conditions. Northstar engine's actually don't overheat not from bad head gaskets (they aren't the defect issue) but the inferior bolts holding the heads to the block. The bolts have been found to be pulling out of the Northstar's soft aluminum block(this is the actual problem but it's blamed on the gaskets). Causing spaces between the block, gaskets and the head that allows hot exhaust air to seep into the coolant flow chambers. This why your have coolant bubbling out of the top of the overflow valve under the hood after only short drives. Under normal operation the engine is literally boiling it's own water supply. The head bolt issue is a design flaw by GM and one that leads to a great number of Northstar engines being crippled at well under 100K miles of driving. Only remedy is to have both heads pulled off the engine, the head bolt replaced with new gasket and you're all set. Bad new very labor intensive whcih = a several thousand dollar repair. I had it happen to my 1998 Concourse Deville. I feel your pain.

The most common overheating problem with the Northstar engine is a blocked purge line. It's job is to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.The hose goes in to the engine to a crossover and comes out the other side to near the thermostat housing,the blockage may be there.

First, anytime a thermostat is overheated, it's ruined.

Second, if this car has electric fans, are they running when they are supposed to? If not, replace the fan control module, and/or test/replace the fan motors.

When the radiator was replaced, were you certain to purge the radiator and engine of all air before replacing the radiator cap? When you refill an emptied system, you always start the engine and watch the fill level in the radiator, refilling as necessary until you know the thermostat has opened. You need to have the heatrer controls set to full hot as well. Once the radiator is filled, put the cap in place, and fill the reservoir to the level indicated for a hot engine.

Finally, check to make sure the lower radiator hose is not collapsing while driving. While this is difficult to check (because your going down the road), just replace it when you replace the thermostat.

All of this assumes you do not have a cracked head or blown head gasket.

These water pumps are know to fail internally. The blade may freespinning. You will need to remove the water pump and inspect.
Or you may have a head gasket issue. Its simple to test if you want a shop to inspect it for you

YOU COULD HAVE A DEFECTIVE WATER PUMP OR THERMOSTAT ALSO RADIATOR PRESSURE CAP.MAKE SURE YOUR WATER PUMP IS BEING TURNED BY DRIVE BELT.NOT SLIPPING.MAKE SURE COOLANT SYSTEM HAS BEEN BLED AND YOU HAVE HALF ANTIFREEZE HALF WATER.CHECK YOUR CRANKCASE WHEN CAR COOLS DOWN.IF OIL STILL LOOK LIKE MILK SHAKE.YOUR NEW HEAD GASKETS LEAKING.THE MECHANIC WHO REPLACED HEAD GASKET DIDNT DECK THE HEADS.THEY PROBABLY NEED SHAVED A LITTLE SO HEAD GASKET WILL SEAL.

It is hard to start because the recent overheat causing a gap between the head gasket reducing the compression ratio in each cylinder hence it is difficult for a combustion.
The most culprit of the car overheating is the poor circulation of coolant due to bad pump or/ and clog up radiator. The 2 important things that have not been mentioned are needed to take care of now.

Its possible your thermostat is stuck in either the on or off position. Also its not an expensive part. I would start there first and go from there. Also how many miles are on the vehicle. Also I assume this is a north star engine.
Thanks,
Lee