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Several possibilities come to mind:
(1) Cooling fan problem? The electric cooling fans should be running long before the vehicle overheats, and always when the AC compressor is turned on.
(2) Proper mixture of antifreeze and water? Having a 50/50 mix is very important. Antifreeze boils at 270 degrees or higher, while water boils at 212 degrees. Cooling fans do not come on on most GM products untill around 220F. Therefore, if you have mostly water in your cooling system, it will boil over long before the cooling fans come on.
(3) Head gasket leak allowing hot exhaust gasses to enter a cooling jacket. There is a quick simple chemical test a garaage can do to check for this if it is suspected. A sign of a head gasket problem would be bubbles appearing in teh radiator neck while the engine is running, a coolant overflow bottle that keeps filling up and not drawing back into the engine when it cools off (because the gas displaces coolant), or if the previous radiator appeared to fail from overpressurization (bulged or failed tanks).
Note also: this engine requires a air bleed procedure to be performed after it is filled, to remove trapped air. If this is not done, the vehicle will overheat. ALso, make sure the system is full.
Here is the fill procedure, and I will post the bleed procedure for you also...
CAUTION: The low coolant indicator lamp may come on after this procedure. After operating the vehicle so that the engine heats up and cools down three times, if the low coolant indicator lamp does NOT go out, or fails to come on at ignition check and coolant is at the level indicated in step 3. If at any time the "TEMP" warning indicator comes on, immediate action is required.NOTE: The freeze protection of the engine coolant after the engine heats up and cools down three times using a thermohydrometer to ensure proper freeze -37°C (-34°F) protection. Obtain the coolant mixture for the inspection from the base of the radiator neck, NOT from the coolant recovery reservoir.
I cannot for some reason seem to locate teh bleed procedure in the databases i have available to me, although the refill procedure mentions a brass bleed valve on this engine.
If your vehicle has air bleed valve<s> they would be found on the water outlet where the top radiator hose attaches, and possibly one on the water pipe in the water pump area. If you have bleed valves, run the engine up to operating temperature with the radiator cap in place, and then slowly open the bleeder screws and wait untill you see a steady stream of cooant flowing out (indicating all air has been purged) before closing it.
I hope this information is helpful; if so, clicking my accept button would be greatly appreciated... thanks!