In most cases is is blown head gasket. The engine suckes air through the hole in the gasket and sends it to cooling system. You never know how long it gonna last.
Can also be caused by a defective pressure relief valve in the coolant reservoir cap. If it is the head gasket, you may also find you can smell combustion gases in the coolant. These won't smell the same as the exhaust gas though as the exhaust has been through the catalytic converter. If changing the cap fails to cure the problem, disconnect and isolate the ht leads or coil low tension wires and remove the pressure cap from the coolant tank. Get someone to turn the engine over on the starter motor while you look into the tank. If you see bubbles emerging in the coolant, it's probably the head gasket.
And a worse case scenario, a cracked head is possible.
Normal causes are low coolant and an inoperative radiator fan.Normal causes are low coolant and an inoperative radiator fan.
Indicates a bad head gasket or a cracked head.
Don't! Most cars have a reservoir for radiator coolant ... add liquid to the reservoir. Opening the radiator cap introduces air into the sealed system ... you don't want air bubbles in the cooling system.
Defective radiator caps or tubes
Find out where you're leaking. Anything that causes the coolant to leak out is cause for concern. Fill the radiator. Leave the radiator cap off and start the engine. Watch for bubbles. If there are quite a few bubbles and the coolant wants to blow out even before the engine gets warm, you might have a head gasket problem.
If your exhaust is getting in your coolant, you have a reeeeeeeeeeeeaaaaaaaaaaaaaaaaallllllllllllllllllyyyyyyyyyyyy major malfunction.
Check to coolant level in the coolant recovery tank and radiator when engine is cold
Possible trany cooler leak
Lack of coolant, lack of coolant flow, air in cooling system, lack of airflow over radiator,........
Loss of coolant with no visible leak. White smoke out the exhaust. Excess pressure and bubbles in radiator.
Broken or split hose Radiator leak heater core leak