Windows Fogging

Anyone else run into the problem of the entire car fogging up badly when the humidity is high ? Maybe it's just because the volume inside a 911 is so much smaller than in a larger car, but it sure seems sensitive. Have to blast the defrost just to defog the windows.

Never.Does it clear up if you turn on the AC?

Yup! Mine does it too. I think it has to do with the foam covering on the Defrost/Heater/Vent flap in the air exchanger unit. Mine is a high milage (for a 911 - 1999/142,000 mi.) car that has seen plenty of use through the years. The foam covering on the diverter flap decomposes over time which exposes large holes in the flap which in turn leads to high moisture air entering/re-entering the cabin when it shouldn't be... I'm not sure of this, and it may be way off line, but I'm going to open things up this winter to see if my foam is indeed gone...

Don't recycle the cabin air, and keep your interior windows very clean. Both help. Window cleaning tips: *dedicated glass only 100% cotton terry towels, no fabric softener *mix in spray bottle 1 part denatured alcohol to 2 parts distilled water *spray towel, wash glass, buff dry with second towel

LexVan is spot on. Recirculated air does not dehumidify and will condense on the windows - more so if they are dirty. I often see loaded cars fogged up on the inside during the winter for this very reason.

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Sometimes. Couple of things: 1) Be sure the body water drains are clear. The ones under the front trunk lid especially. If these are not clear they can retain water and this can add to the humidity -- at least initially -- the A/C system pulls into the cabin. While I am on the subject be sure the cabin is dry. Check the carpets on the floor and along the door bottoms. I found the passenger side door bottom carpet wet on my Boxster the other day and the problem appears to have been a door membrane that was not sealed properly. 2) If you use the A/C try to remember to a few miles/minutes before your final destination to turn off the A/C and just let the cabin vent fan run to dry some of the moisture from the system. If you don't do this upon restart the moisture that was present can result in a sudden fogging of the car's windows. 'course, some times when one does this the glass starts to fog up again so you have to "time" this so you maximize the drying time without running into a fogging window problem. 3) Be sure the cabin air filter is not real dirty and is installed correctly and is not damp. 4) I have found the inside glass surface develops a film of a mixture of I guess fine dust and who knows what else. (I do not smoke and no one else smokes in my cars.) Even though I practice (1), (2), and (3) above occasionally it seems the fogging problem gets worse and then I clean the windows and things are better. In my area about twice a year is par for the course for interior glass cleaning but depending upon where you live/drive you may need to do this more (or less?) often.

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Item # 2 - The A/C is in-itself a "dehumidifier"...Ever notice that the AC goes on when you hit the Defrost button?...The "cabin vent fan" will not dry the cabin air...

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Bingo! Those pesky drains will do it. I think that the amount of moisture that is inside of the car is much more than would be "hitching a ride" on the air. Look for water incursion inside of your car.

Thanks for the feedback. Everything inside the car is dry. I've tried turning the ac off with a few miles left to drive, but this doesn't seem to do the trick and I agree that the ac should actually dry things out. I have had a lot of the black foam which has degraded come out of my air vents, so maybe there is something related to this. I like the idea of checking the drain areas, since this seems to make the most sense, especially since this seems to happen more after it's rained.

Sooooooo..... do we have any good solutions to the foam issues that we all seem to have or will have at some point? Is their a way to change out the foam easily?

just leave the windows open

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+1 This also makes the car magically sound better!!

Unfortunately it's a bit chilly up here in MN this time of year for the open windows, though I do agree it sounds good.

seat heaters and a warm jacket would do the trick

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The "cabin vent fan" blowing HEATED airflow will ALWAYS clear the interior surface of the windshield of misting... whereas, A/C, only maybe But... Ask yourself.. What happens once the A/C is shut down, manually or automatically (OAT declines below 35dF) with the condenser CLOGGED with condensate. Best not to use A/C except for summertime cooling needs. HEAT will always clear the windshield of condensate. A/C dehumidifying capability depends on dewpoint and OAT. As OAT declines below ~45dF A/C dehumidifying capability declines, non-existent below 32dF.

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Yes, I'm well aware the A/C dehumidifies the air. What I was referring to is the the A/C evaporator gets wet as it is very cold when the A/C compressor is running and the moisture in the incoming air condenses on it. If one shuts off the A/C without running the fan the evaporator stays wet and can develop an offensive odor. And if one gets back in the car even the next day -- which has been my experience -- upon engine start and the cabin fan begins to blow the inside glass surfaces can develop a fog faster than one can say scat. By turning off the A/C but letting the cabin vent fan run this dries the evaporator and as the moisture leaves that and then hits the cold air passages and condenses it will dry these out. The downside is in some weather conditions the inside glass surfaces can begin to fog up. More than once I've had to turn on the A/C to clear the fog then turn off the A/C to dry the system.

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My experience is with the A/C running this defogs the inside glass quicker than just air blowing on it because the air that is being blown out the vents has had its moisture content lowered. The A/C system has a temperature sensor -- probably uses the outside air temperature sensor -- and if the outside air temperature gets too low the A/C compressor is turned off to avoid the evaporator getting too cold to the point the water that has condensed on it freezes and blocking air flow through the thing.

Does anyone know how many drain holes there are ? I think I found 2 of them near the front of the hood, but it's tough to really see much. How would you make sure they are clear ? Running some water thru them and seeing if it drains ?

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I believe there are 3 under the front trunk lid. One on each side of the battery box and one under or very close to under the battery box. You have to remove the battery cover -- you can leave the battery alone -- and the cover on each side of the battery. This exposes the 2 outliers and with a good flashlight you should see the 3rd one. The manual calls for these to be "cleaned" the drains at any rate with compressed air. I never had used compressed air. I remove by hand all the trash then take a pitcher of water and gently pour into the water catch basin and ensure the water drains right out. If it doesn't -- and so far with my cars is always has -- one might be able to take a suitable diameter and length of a rubber hose and gently run it down the drain hole to act as a gentle (there's that word again) drain rooter. I stress gentleness as you do not want to damage the water catch basins or the drain hose nor do you want to knock the hoses loose from the drain basins.

Do you smell antifreeze in the car? If so maybe time to check the heater core for leaks.

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It can take up to 20-30 minutes, depending on OAT, to raise the evaporator core temperature enough that the condensate begins to evaporate, and now getting it fully exhausted from the cabin, another 20-30 minutes. Best to leave the A/C OFF, use HEAT!

I just drop the top and voila!!!!!! Sent from my iPhone using Rennlistedtrtfytj

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Hmmm, while I do understand the theory behind the BS you're spewing, I don't need to "ask myself" anything ...When the windshield starts to fog up, I hit the defog button on the top left side of the climate cluster. When I do this, the defog symbol lights up as well as the little snowflake symbol (that represents the A/C in case you don't know). In addition to these two little symbols the fans also come on full blast. From this point forward it usually takes about 5 - 10 seconds to defog the windshield. If I manually shut the A/C off and leave the fans/heat on, the windsheild stays foggy. That's called annecdotal evidence (in case you didn't know) that the A/C in conjunction with the defog/defrost function, does indeed clear the windsheild. While I've never experienced foam coming from my vents, it's my hypothesis that the foam on my mixer flap is gone (my car has had 2 previous owners), and that's why my windsheild fogs so often/quickly.

I got rather tired of my windshield fogging suddenly and spontaneously and added a switch to open the A/C compressor circuit. Once you make use of the A/C you cannot turn it off(***) absent raising the cabin Rh to the point the windshield fogs over once again, unless you heated the windshield in the interim. ***: You just drove into a cold zone and the A/C was shut down automatically... SURPRISE sudden windshield fogging.. be quick with the HEAT! Which 996 do you have? My 2001, once the cabin has acclimated to the temperature seating, at least to within 2-3 degrees, does nothing other that route more (coolish) airflow to the windshield.

I cover the defroster vents (driver side only) with a paper towel actually, & it prevents my windshield from fogging. Only happens in the am on way to work, when I get there, I remover said paper towels as it looks rather tacky, lol.

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Methinks your post be about exterior windshield fogging on a humid day. My solution is to manually cycle the windshield vent open and then close. I have no idea why automotive A/C designers have chosen to CHILL the windshield on a hot day. Or is it fogging the morning following the previous day's use of the A/C... In which case the same procedure will work.

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I am guessing it is due to humity as you say. I'll try the recycle trick the next time it happesns, but it probably won't for quite some time now.

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The idea is that once you start the car you take windshield vent selection out of automatic control by switching it open and then closed. The same procedure can be used to prevent the early morning windshield interior fogging episodes.

Is the windshield vent "blowing" when the AC is on, because that's what causes it.

Yes, and that's a big part of the problem. My guess is that there was a desire to widely disperse the cooling airflow such that driver/passengers are not discomforted. So, automatically, all three flow routes are open to flow once the cabin acclimates to the temperature setpoint.

My windshield started fogging almost immediately after leaving my garage.

In Memphis even my eyeglasses often do that.

I find it interesting that on many other cars you can have the air flow settings so that the windshield is in defrost mode and you are able to get heat to the feet as well, yet on a 996 this doesn't seem to be an option.

I am able to direct air flow, I believe, 4 different ways.

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I can assure you that the entire Toyota/Lexus product line exhibits the same problem. I suspect it started in the late eighties with the design effort to minimize the space taken up by the Heat, A/C plenum, in order to accommodate all the additional electronics/airbags, etc, etc. I first noticed this in my '92 Lexus, not to surprised to discover it in my '01 C4. I think it to be criminal that when you ask for windshield demisting with OAT below 35dF (A/C automatically disabled) what you get is a mode change to "fresh" (if needed), singular airflow path to the windshield, 68dF(***) airflow, less than moderate airflow rate. ***: Assuming a cabin temperature setpoint of 72dF and has already acclimated at that level. In my '92 Lexus I modified the IAT thermistor circuit such that it "signaled" a 45dF cabin temperature whenever I selected defog/defrost. Select demist: HEAT & MAXIMUM blower speed! In my '01 C4 the A/C compressor clutch circuit is open except for actual cooling need.

Does eat OATs and Mares eat OATs and little lambs eat ivy... Just hit the defrost button, it works...

Hello In my part of the world Bergen, Norway . The wettest and most rainy place in Europe. This is something that most people experience here from time to time. The cause of the problem is that there is a temperature difference from inside to the outside of the cabin. If the inside is warmer, it can hold more humidity in the air inside the cabin, basic thermodynamics. When warmer air carrying humidity meets a cold surface condensation appears on the cold surface because the temperature of the air carrying the humidity drops, resulting in it discarding some of it's humidity. To minimize this effect one have two options, one: keep the temperature of the inside of the cabin the same as the outside. Or two: always keep the inside of the car dry. I opt for number two of course as it's normally quite cold here. Recommend buying 997 rubber floor mats (not available for 996), they do wonders, and you see how much water your actually bring into the car with your shoes in the form of a puddle on the mats (in my case). It's difficult to believe but when the floor mats hider the water soaking into the carpets you see just how much water you drag into the cabin.

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Third option: Keep the interior surface of the windshield WARM! Keeping the cabin (atmosphere) dry.... Remember that a big part of this equation rests on human metabolism, breathing and perspiration. The solution for that is air flow through, OUT of the cabin, NEVER use recirculate mode in the winter months, and it not a bad idea to manually control/set the blower to a higher speed(/pressure) than that which would be selected automatically... Forced exit airflow, even lowering the windows ever so slightly.