Coolant boiling out of overflow

Need to check 4 things,
As their are no details about year or model I assume it is pre 80's and old school mechanical diagnostic is called for.
1. Condition of radiator fins be sure they are not blocked
2. Check flow of coolant May be a blocked thermostat.
3. Check condition of engine - oil too low can cause overheating.
4, Check level of coolant in radiator tank as opposed to coolant overflow. If you have a small leak, bad radiator cap or head gasket problem the expansion system will not replace coolant to tank allowing it to boil dry.

You have a coolant leak. The engine gets hot and pushes coolant out under pressure into the overflow tank.When it cools, the volume of coolant decreases, the pressure drops to the point that it becomes a vacuum. The vacuum pulls in air more easily than coolant. It's thinner. So, you keep losing coolant from the radiator which makes the engine run hotter, each time until it eventually overheats and does damage. You need to have the cooling system pressure tested. The leak will show up. Usually, it's a loose hose clamp or cracked radiator tank high up, which makes it had to see the coolant because it doesn't leak much and it boils off leaving little or no evidence. Fill the system completely before pressure testing. Remember, checking the overflow tank means nothing when checking coolant level. There's no short-cut to checking the coolant level, take the radiator cap off when cold to accurately check level. Read the safety precautions on the car but always check when the engine is cold. NEVER HOT.

is the cooling fan coming on when it is hot?

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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

Either the thermostat is stuck in the closed position...or the engine cooling system cannot pressurize...both will cause an overheat condition.
A blown head gasket can cause overheat condition as well as fluid loss (boil over)

I'd maybe remove the thermostat and either just replace it or not have a thermostat installed just to see if it still boils over. If it boils over with no thermostat installed then you're getting into possible leak in cooling system...i.e. water pump weep hole, hose or radiator leak...or a blown head gasket.

Hope this helps...

Depending on your system (either sealled radiator without a cap and a pressurized surge tank or standard radiator with cap and an overflow tank) either the radiator/surge tank cap is not holding pressure and the coolant is unable to pressurized and boils at a lower temp, boiling off until you coolant level is so low the engine overheats. More than likely it is either the thermostat or water pump. Commonly it is the thermostat

head gasket failure between piston seal and the cooling system causing pressure ,it will not show as water in oil but white smoke on start up from cold ,when it warms up it will tend to seal itself but eventually it will just keep boiling up

head gasket or water pump impeller has come loose on the drive shaft ,its plastic on a metal shaft

check to see if electric fans turn on in front of radiator