It sounds as if it is boiling in the over flow tank top and bottom hoses are hard from pressure

I think that you cooling fan do not start, May be the thermostat or the cooling fan do not working find, please, check both.
Regards

Could be a defective radiator cap, or a leaking head gasket.

Check the radiator-it may have blockage. Take it off and turn upside down and flush from the bottom.
If the top hose gets hot, that means the thermostat is opening. The bottom hose should be at least warm, and quite warm if radiator flow is good.

Well sounds like you are overfilling the tank. Secondly you would loose pressure and probably over heat if it was your cap. Also check top and bottom hoses going to radiator after it has come to operating temp. If the thermastat has'nt opened one of them will be rock hard. Perferably it be the top hose hard. Check for leaks. Also the overfill tank is going to look low at times cause the engine heats up and water flows into the radiator and when the thermastat closes it back flows back into resorvior making it look low when it is flowing. Also need more input on why this is a problem.

The hard hose it a sign of max pressure in the cooling system. Check that the fins in the core of the radiator are solid and not blocked. have a pressure test done on the cooling system for engine faults. Have the radiator fushed out by a radiator shop. Check that the thermostst is opening. Start cold and hold you hand on the top hose It will be cold for maybe 10 minutes and if the thermostat is working it will get warm and then hot to the touch as the unit opens and allows hot water to flow to the top tank

you have a blown head gasket.. doesn't have to have coolant in the oil for a blown head gasket... if hoses are rock hard when running then you have exhaust leaking into the coolant via a blown head gasket...

Answer could be a faulty thermostat,or it could be a sign that your head gasket is about to blow ,or last of all your timing could be out of sync.

Answer 2 from Inventus: It means your cooling system is funtioning properly. In a system having a coolant recovery tank, coolant in the radiator is always up to the brim, hot or cold. There is negligible or no air space. When coolant in the radiator expands sufficiently due to warming from the engine, it will squeeze past the pressure cap's bottom seal and flow into the recovery tank. (If no provision for such expansion was present, the expansion would rupture the radiator or your hoses.) Only coolant within the radiator is under pressure, and because of this pressure (together with the elevated boiling point that the "anti-freeze" permits), it normally does not boil. But once past the pressure cap's bottom seal, the overflow is at atmospheric pressure and therefore boils.
This boiling is usually unnoticed after a short, i.e., local, trip because the cooler coolant already in the recovery tank quenches it. But after some highway driving the influx of more hot overflow heats up all the coolant in the recovery tank to the (unpressurized, i.e., "natural") boiling point.
As the engine cools when shut off, the contracting coolant in the radiator sucks back coolant from the recovery tank. Fluid in the recovery tank should never be below the "full hot" or "full cold" marks, lest air be sucked in.
-- BETTER ANSWER ==
Your cooling fans are not turning on. It is not normal for your overflow tank to boil like that. It is true that your radiator is overflowing into the reserve tank, but that means yourr adiaotor is boiling. Check for blown fuses or relays for your cooling fans. IF theya re fine. run your engine for about 15 minutes and drive. When you temp level is at normal operating temp open your hood with the engine runing and see if your fans are on. If they are, then you may have a bad thermostat or a plugged radiator, or a bad water pump. If the fans are not on, get your cooling fan switch replaced if your car has one. Check your temp sending sensor

sounds like a blown headgasket. when it is cold and you start it squeeze a radiator hose while it is cold and if it is hard(has pressure) then head gasket. also check for white milky antifreeze. CAUTION cooling systems are under pressure and contain boiling hot water. If you are not experienced do not attempt to fool with the radiator /cooling system. blown gasket repair $400-600

exhaust gas coming thru the resiovoir, sounds like a head gasket

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.
If the purge line is blocked you may have an air lock preventing coolant flow.
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.

Air locks in the Northstar engine shouldn't be a problem as the system is self purging, providing all the hoses are clear.

Check the coolant level in the tank,it should be half way when cold.
There is no cap on the radiator as it is a fully pressurized system.

The expansion tank is the only means by which to add coolant to the Northstar engine.

Check to see if your fans are working by switching on the air con,they should run all the time.

With the air con off,the fans won't come on until the temp hits around 225 degrees.

Also, if it's boiling over, the cap on the expansion tank may not be holding pressure and therefore needs replacing.....a new cap only costs a few dollars so it is worth replacing to see if this solves your problem.