The most common overheating problem with the Northstar engine is a blocked purge line(small hose).The purge line is designed to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank (not the cap) ,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 close to the thermostat housing,the blockage may be there.
Some model vehicles are equipped with two electric cooling fans. Only one will operate when the a/c is off. Generally, when the a/c is switched on, the second fan will operate. It is designed that way to draw more air through radiator. In this case, you stated that the car overheats only when the a/c is switched off. That would indicate to me that the other cooling fan (as mentioned above) is not operating at all. Check the cooling fan relay and or fuse in the main box under hood. It could be there is a blown fuse or relay, or the fan motor is bad. Please keep me informed as to what you discover.
Testimonial: "what if both fans are on?"
With a/c off, does it overheat when you are stopped or moving? If it overheats when stopped and both fans are on, then it could be a clogged radiator. Sometimes the fins in the radiator become clogged with dirt and bugs. I take it the thermostat is good, otherwise it would overheat with or without the a/c being on or off.
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