Question about 1999 Pontiac Grand Am GT

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99 Grand Am GT windows fog up

In certain conditions the windows fog up, but there is NO coolant loss
or odors, so I don't think it is the heater core. Winter time is the worst.
I can open a side window a little and it will clear up. It seems to be
better when the A/C is run, but not always. The problem is not consistent. Running full defrost clears the windshield, but not the side
and rear, Running the side vents helps too, but only partially. I am
thinking maybe a damper is stuck or air flow from the outside is not
circulating correctly. Any help would be appreciated.

Thank You

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Fog on the windows is caused by moisture in the air condensing on the cold glass. You probably knew that. So the solution to your problem is to reduce the humidity in the car or warm the glass above the dewpoint. Running the AC will help because any moisture in the air will condense on the much colder evaporator coils before it gets to the windows, but as soon as you turn the AC off that moisture goes right back into the air. Fresh air from outside will help if the moisture level in the outside air is lower, which it ussually is in the winter. The only sure solution is to warm the glass.
You may want to check a service manual and see if there is an air filter somewhere in the cabin heat plenum. If so you may want to replace it. Defrosters and side vents should provide plenty of warm air to clear all windows and it sounds like your just not getting enough air flow. If you're not really into getting behind the dash and tinkering with the heater plenum, you might consider a small 12 volt heater like the ones available at auto parts stores, or you could just let the cabin warm up well before driving. There are also glass treatments that you can use to prevent fogging, I used one for years on a motorcycle helmet, These are ussually a wax and or soap base so they have to be cleaned off and reapplied periodically, but they do work well.

Posted on Nov 15, 2008


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97 Ford Taurus

Replace the engine thermostat, it is probably stuck open.

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At engine operating temperature, it opens up, sends the coolant to the rad, and keeps the engine coolant at that constant temperature.

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Thermostat replacement is an easy, quick job and cheap.

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