Antifreeze smoke coming out heater vents

Your heater core is leaking steamed antifreeze into the vents and possible out the firewall,where the heater hoses connect to it. Have that checked out.

Sounds like a bad heater core. You will get steam/vapor out of vents from this. If you are referring to the over flow tube being the evaporator drain tube at the firewall its because the heater core shares the same case with the A/C and its filling up with anti-freeze.

sounds like your heater core is there any resadue on your windsheild if so then its your heater core

often that smoke is actually misty antifreeze from a leaking heater core. wipe a little of it up and sniff. sickly sweet smell it is heater core time.

This sounds like a blown heater core or heater hose. If you smell antifreeze and/or see a puddle under the car or on the floorboard, you would most likely have a heater core gone. This can be a time consuming repair due to the fact that the core will normally have to come out under the dash area towards the floor.

You have a leaking heater core. Leaking antifreeze into the heater case. Soon you will have a puddle on the passengers side floor.

The fluctuation in the temp gage indicates a low coolant situation. The smoke you see from the defroster vent is actually antifreeze that is leaking from the heater core somewhere and as the antifreeze evaporates from the heat it causes a vapor to produce. You should smell a very sweet smell when you see the vapor. If so it is definitely antifreeze. Find the leak and fix it. Then refill your coolant system to specs and you should be good to go.

if it smells like sweet antifreeze, it is
the heater core is not he A/C evaporator core.

the core fails due to not replacing the antifreeze coolant before it goes acidic ... (not doing service here)

are you getting leakage under the dash near heater box, blower area?

Heater core or one of the hoses are leaking. Low on antifreeze now so no heat.

You have a coolant leak most likely in the heater core. Try disconnecting your heater hoses and coupling them together, then running the engine to see if the leak goes away. If it does, I would have the heater core replaced.