Foggy windows

My wife drives a 2001 Chevy Venture. In cold weather, while driving at speeds around 35 mph and up, her side windows and edges of front windshield fog up despite having the defrost cranked up as high as it goes. When she slows down to 20-30 mph or stops, the windows will clear up. She took it into the dealer and they said everything checks out. What can we do?

I think the HVAC system may be switching itself into “recirculate” mode and keeping the moist air inside the car. When the car slows down the system goes back to “fresh air” and the dry air from outside clears the windows.

If the system is vacuum operated this would make sense, since engine vacuum is highest when the engine is idling and drops off as the throttle is opened. A vacuum leak in the HVAC system could do this.

If the system is electrically operated, the vacuum idea doesn’t hold up. I don’t know which system your Venture uses.

Does the AC compressor work when the defroster is on? It should, and this should help de-humidify the air.

Thanks for the reply. We’ll check this out.

Hi folks, I bought a used 2002 montana in december when it was very cold. Windows fogged up so much I couldn’t see out the side windows at all. No smell of coolent. After spending many hours of investigation, I finally discovered the problem. In the back of the glove box there is a compartment which contains air filters. I removed the air filters and looking inside with a flashlite I saw in the very back, a little higher on the firewall, a plastic door which was shut. I reached in and gently opened it. Now fresh air gets in and NO more FOG. I hope I just saved you some time so you can spend it with your family. -Andy

In addition to the other suggestions, I want to add that a clogged cabin air filter will result in fogging of the windows. Has the cabin air filter been replaced, as per GM’s maintenance schedule?

I also have a 2003 and window fogging was awful.After reading some posts i removed the filters. Windows were about 50% better. I then reached in and opened the small door mentioned in this post. The other day i travelled 4 hours in temperatures of -32 celcius and no fogging at all. Thanks for the solution