What causes bubbles in the radiator coolant?

Answer 1

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.