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Recently there has been a lot of fog on all of the windows in the car, particularly on my windshield. It's been bad enough that I've had to stop and wipe the glass off, but the fog can come back during a drive.

What can cause all of this fog on the windshield? What things can I try myself, before taking it to the shop?

I have a 2002 Subaru Forester.

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3 Answers 3

up vote 7 down vote accepted

Typically the fog is from excessive humidity in the vehicle. The three most common sources I can think of are:

  • A leak that allows rain water to collect in the car. Check to see if the rugs are damp, look in the storage well behind the seat and any place water might collect.
  • Do you also notice a sweet smell when the engine gets warm? If you do it may be a leaking heater core. Antifreeze could be leaking into the heater box then the vapor is fogging the windshield. Antifreeze residue can be difficult to remove from the windshield. Try rubbing a clean tissue on the inside of the glass. The tissue may show the oily component of the antifreeze.It may be a small leak as a little antifreeze goes along way in terms of fogging the glass.
  • The third source could be the A/C. The A/C unit removes humidity from the air. This is what causes the puddle under the car when the A/C is running. Sometimes the drain get clogged with leaves or pine needles. This allows the water to build up which adds humidity when you run the defroster. If enough has collected you may hear it sloshing under the dash when you make a turn.
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I would add to this - if you have cleaned the inside with products which leave a residue of any kind, you may need to give it a proper clean to get rid of this. –  Rory Alsop Nov 3 '12 at 1:09
2  
We did notice an odor - but it smelled more like celery than "sweet". I had a guy replace the windshield (It needed to be done and I thought it might be leaking.) He said it definitely smelled like antifreeze; he also pointed out a rainbow-colored sheen visible from the outside on the windshield, which was probably antifreeze. It's important to note that the fog is significantly lessened when I've left the windows cracked and parked under cover, as well as by avoiding using the heater at all. This allows me to make short trips until the car goes into the shop on Monday. –  KatieK Nov 4 '12 at 4:59
    
Sounds like a small leak in the heater core or connections. Let us know what you find out on Monday. –  NitroxDM Nov 4 '12 at 17:50
    
Yep - the shop says it's a leak in the heater core. Boat payment! –  KatieK Nov 6 '12 at 3:35

If you have an a/c, you can leave it on even if you have the heat on and it will clear the fog. Wipe down the inside of your windows with windex, that will also help.

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You don't have the air inlet selection lever set to recirculate, do you?

enter image description here

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Nope - I'm pulling in outside air. –  KatieK Nov 5 '12 at 18:36
    
I'll leave this answer up here for the next person. I had a friend who kept a windshield-wiping rag on his dash for months until he realized what this switch did. –  Jeremy Stein Nov 5 '12 at 18:39

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