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Yesterday I was driving my 97 Accord down the highway with the air conditioning turned on. It started raining heavily for a little over 5 minutes, and then the rain stopped. Suddenly, what appeared to be whitish smoke started coming out of the 2 center vents. I quickly turned off the air conditioning, and the smoke stopped coming out. After around 10 seconds, I turned the air conditioning back on and no smoke came out. I drove for another 30 minutes and didn't see any smoke again.

I have never seen this happen in any car in my life, prior to yesterday. Also, it may not have been smoke, because I did not smell anything - so maybe it was water vapor? None of the warning lights on the dashboard lit up. Also, I had the ventilation system set to "recirculate" and was not using the defroster. Any idea what might have happened?

Edit: On 2 occasions in the past week, on sunny days, I have seen similar white smoke with no odor (although not quite as thick as what I described originally) coming out of the vents for about 10-15 seconds with the AC turned on.

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  • Frozen water vapor is my guess.
    – Moab
    Jun 10, 2018 at 15:07
  • Much more likely just plain water vapor, aka fog, not smoke. AC vent output temp is rarely below 50 F.
    – blacksmith37
    Jun 29, 2018 at 16:02
  • When I lived in the midwest with high humidity, I noticed the same phenomenon on occasion. It's water vapor. All is well.
    – Dacid Salin
    Sep 28, 2018 at 4:05

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You did not indicate any foul smell, which eliminates a coolant leak, as the odor would be obvious. I believe the key factor is that you've described a heavy rain. If your ventilation system was set to allow outside air, rather than recirculation, it's likely that some of the high humidity (heavy rain) was pulled into the ventilation system.

It would have been sufficiently heavy volume to condense from the cooling of the air conditioning system and present itself as your own personal cloud.

I have experienced this effect in the same conditions as you describe. To prevent it in the future, when you encounter heavy rain, switch to recirculated air until you exit the area, or you can choose to ignore or revel in your temporary cloud with a silver lining (no troubles).

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  • Yup, it's water vapor. While refrigerant may not have that much of an odor, if he had that big of an evaporator leak 1)it would have emptied the system in a few minutes and the AC would have been immediately cut off by the pressure sensor or low pressure switch 2)He would definitely have smelt the refrigerant oil (it's meant to be soluble with the refrigerant so that it won't pool inside a part of the system, depriving the compressor of lubrication).
    – Al_
    Jun 10, 2018 at 16:32
  • My ventilation system was set to use recirculated air (I rarely have it set to outside air).
    – pacoverflow
    Jun 10, 2018 at 16:32
  • It's probably an automated system with no manual actuator for the recirculation flap. Even though you set it to recirculated air, it should let in some fresh air from time to time, or after a certain amount of time (such as 10 minutes).
    – Al_
    Jun 10, 2018 at 16:35
  • I suppose it's possible that the heavy rain provided cooling in the a/c system beyond the usual low temperatures, which caused additional condensation from the air. That answer may make more sense as it explains how the condensate would not collect in the drip pan and exit the vehicle in the usual manner. Your inside air would still contain humidity, would possibly contain excessive humidity and the extra cooling turned it into a cloud. This sort of experience is difficult to create intentionally.
    – fred_dot_u
    Jun 10, 2018 at 16:36
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    @Al_, additionally, it's possible that the defroster was selected which works more effectively with outside air being allowed into the vehicle.
    – fred_dot_u
    Jun 10, 2018 at 16:43
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I had a similar issue happen to me, it wasn't raining though and it wasn't on the highway

I was driving back from my uncles house that's about 20 miles out of town, once I got in town I stopped at the first red light and all of a sudden, white smoke started pouring out of the vents, I immediately turned off the road, popped my hood and stepped out and white smoke was pouring out of the cabin, I waited a few minutes after checking everything and then started it back up, everything was fine, all until I got home. It started up again

I took it to the Mechanic the next day thinking "oh no, what's this going to cost for a new heater core" he had it for 5 minutes and fixed the issue

It was the coolant resovoir cap, one the seal was bad, and two the threads were stripped. New tank, new cap. Never happened again.

I'll tell you what though that sudden dense white smoke filling the cabin about makes your heart drop.

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