What is wrong when the coolant tank starts bubbling after driving your vehicle for a while?
I assume you mean that the coolant is boiling up into and out of the overflow bottle.(picture is of a fancy stainless steel bottle … I have never owned a car with anything other than a plastic one, but they look pretty much like that … and who knew finding a decent photo would be so difficult)So why is your coolant boiling over? Could be:Cooling fan is not running - because:motor brokerelay brokefuse is brokebelt is slipping (if you happen to have a mechanical fan instead of an eletrical one)Water pump is not working - because:motor brokerelay brokefuse is broke (if you happen to have an electrical water pump).belt is slippingRadiator is gacked / corroded and no longer flowingInsufficient coolant - so you don’t have enough material to exchange enough heat … and what you do have is getting boiled … so where did the coolant go? You have a leak somewhere?water pumpradiatorhoseoverflow bottleBad coolant mix (either way too much anti-freeze or none) - this seems unlikely as a sudden onset thing (the chemistry in your cooling system doesn’t just change overnight and many cars will run plenty cool on just water).Cracked head gasket - although you should see coolant / water in the oil and probably be getting smoke out out of the tailpipe (you can usually smell it too) - depends on how bad the crack is I supposeThermostat is broken - this is the thermostat that is inside the engine, and used to either pass collant through the engine block or shunt it from the block (this doesn’t usually present as a boil-over but it could - and it is usually cheap to check and relatively easy fix).All of these things can cause a coolant over-temp … and hence the boil-over into the overflow bottle.