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1997 Jeep Grand Cherokee windshield fogs up for no reason

My jeeps windshield is fogging up for no apparent reason. It happens at all different times of day and even while driving down the road. If I turn my heater on on defrost it disappears quickly but returns as soon as I shut it off. It is still too warm to be driving around with the heat on in fact ive got windows open. Is there a root cause to this issue just started happening in the last week or 2.

Inside or outside of the windshield?

Outside I believe but haven’t actually checked that

Usually when it fogs up like this,water is getting in.Check around window seals and around windshield.Check your carpet for any wet spots.

Have someone check and see if the condensate drain tube for the AC is restricted.

Tester

If the fog was on the outside of the windshield you would be able to clear it with the wipers. I think the fog is on the inside and the heater core is leaking.

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Was my first guess also, unless the fog is on the outside.

I have a similar problem on my Forester. In a light rain condition, the rear window fogs up on the outside, but only on the area covered by the rear wiper. I am puzzled by that. The defroster does little good. I can keep the wipers running, but why is the area not covered by the wipers fine, no fogging up?

If it’s on the outside, the windshield’s temperature is below the dew point. If it’s on the inside, you have a moisture buildup inside the vehicle. Do not use recirculate mode for the HVAC system. If you smell antifreeze, the heater core is leaking.

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