What would cause white smoke to come out of heater vents?

Most likely the heater core is leaking. If there is also water on the floor and an antifreeze smell in the passenger compartment, those are also signs of a cracked heater core. Also, GET THAT TEMP GAUGE FIXED!!! You could be overheating and not even know it. Once the motor overheats, you're risking a blown head gasket, a VERY EXPENSIVE repair, or even worse, a blown engine. If steam is coming out of the vents it is very possible the motor is overheating. You need two repairs, your heater core and your temperature gauge.

My heater core was replaced since it had a hole in it. The blower was also replaced but with a used one. The guage is now working and so far, no more white smoke. Heater does not get hot though and the blower motor is very loud. Hey, ya gotta laugh a little...from one thing to another. I suppose the worse thing is over. Thank the Lord though I did not have a new car since the guy that worked on it did a number on my dash.

Symptom: on a 2000 Ford Contour, white smoke came through the vents and the smeel was acrid. We switched the car off. A few hours later, the car was fine, no more smoke, but the heater didn't work on one speed setting.

Solution: there is a resistor block which controls the blower speed in the ducting. When one of the resistors goes, white smoke is fed dirrectly up to the vents. Replacement is easy (admittedly while doing a head stand in the passenger footwell) and fairly cheap.

Suggestion: many other cars will have a similar layout. Check this out before investing in a new heater core...