2013 Passat TDI Windows steam in snow

Palouse said:

When the AC activates in defrost mode only the dehumidifier is working--it is not cooling.
When I know I'm about to experience a wild swing in humidity and temperature and lots in getting in/out of the car, I just set the HVAC on auto and ignore it. VW engineering does the rest very, very well.

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Uh, sorry, but dehumidification is done by cooling - it condenses out the moisture. Modern systems are typically reheat - heat after cool, so the air is chilled to dehumidify, and then heated, which is why you don't feel the cooling.

- Tim

tadawson said:

Uh, sorry, but dehumidification is done by cooling - it condenses out to moisture. Modern systems are typically reheat - heat after cool, so the air is chilled to dehumidify, and then heated, which is why you don't feel the cooling.

- Tim

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^^ This ^^

Tim'a right on. The only way you dry the air is to precipitate the water out (lowers dew point), which occurs when it condenses on the cold evaporator coils. Heat may be added on the post treatment side to warm the air and maintain interior temps (which lowers the relative humidity even more)..

Actually warm air is capable of holding more moisture than cold. Much more. Thus, by warming cold air we reduce its relative humidity. In theory everything should be fine... That is until the warm air hits a cold object and the absorbed moisture condenses out onto the object in question.

Now comes the conundrum. In summer it's best to cool incoming air immediately. Thus cooling fins before anything else. It works great in summer because a bit of heat can be added to already dried air on somewhat cooler days. It defogs windows nicely and makes the occupants happy... and dry.

In winter we have another situation entirely. Cold air, often below freezing, at high humidity. Then wet feet and mats. Next a passenger and maybe more... All of which are moisture machines. Pass cold moisture-laden air over colder coils and about the best one can hope for is snow. Pass somewhat warmer air over below freezing coils and coil icing becomes a problem. So the problem becomes a matter of warming the air enough to cool it and condense moisture onto the a/c coils without freezing them solid.

Cabin defogging in colder weather becomes a balancing act. Cabin air must rise above freezing by a margin. Coils need to also remain above freezing but colder than ambient air. Cabin heat must work in conjunction with the cooling in order to optimize ridding the vehicle of moisture instead of fogging.

For this reason it's not unheard of to recirculate air for a few minutes as the vehicle warms up. Cabin air at progressively warmer temps will be passed over a/c coils and the moisture removed. The dry air will be reheated and passed into the cabin where it will absorb more ambient moisture and the cycle will repeat with a resulting dry cabin and no fogging.

Remember that a good part of the "problem" is the fact that these cars are so tight. Arctic breezes blowing through a vehicle are much easier to deal with. In other words you pick up cold air at a high dew point but low actual moisture level. You then heat the daylights out of the incoming air, pass it into the cabin, and then expel it from any number of cabin leaks. Not a bad plan if you don't mind some wind noise.

Understand the problem and you'll deal with it nicely.

How do you remove the cowl at the base of the windshield?

Remove the wiper arms. Remove the rubber sealing strip at the front of the cowl then "unzip" the cowl from the groove at the base of the windscreen.
Yours
Vern

I have a 2013 SEL Prem. My build date is 8/2012. I have the drains and perforated cowl, which is unsealed to the bottom of the hood.

Man I wish the diagrams in the repair PDFs I have from Erwin where color coded like that.

I have a 2013 SE with a build date of 5/13. It has the non-perforated cowl cover and it has a big problem with fogging the windows. I didn't notice it until the last month or so when the temperatures dropped. I drove in freezing rain for 4 hours the other day and had to actually put my side window down a crack and turn the heat and A/C nearly all the way up to keep the windows clean. Last night it was not raining or damp but I drove somewhere in 30 degree weather and got back in the car after a couple hours and the temp had dropped to 20. The interior side of the windows were fogged and iced and it took 6-7 miles with defrost on high for the windows to clear. I never had this problem in the 2012 I drove last winter. Looks like I might be doing some modifications in the near future. But first I guess I ought to make some noise to VW so that hopefully they will correct this in future designs.

2/13 build, non-perforated cowl.

I looked around the engine bay and it seems like if you just simply cover the holes in the fender liner itself, the openings can still pull air from the engine bay. While it's not optimal (especially if you have some odors from leaks), it should still be better. I looked under the cowling and can't where the intakes would be if the water drains themselves were completely plugged.

I have a 2013 which I bought in nov 2012. I live in Georgia and get virtually no snow. On several occasions when driving to work on rainy days, my windshield will fog up half of the windshield and much of the front door windows. No matter what I do, the fog will not go away, no matter what setting I use. I took it to the dealer and they said they had no idea what it was.

Wish there was something I could do.

Bobmws said:

Every other vehicle I've owned that was equipped with factory AC automatically activated the AC when in the defrost mode.

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X2

I think its weird I have to push the AC button to defog the front windshield.

SilentShooter said:

I have a 2013 which I bought in nov 2012. I live in Georgia and get virtually no snow. On several occasions when driving to work on rainy days, my windshield will fog up half of the windshield and much of the front door windows. No matter what I do, the fog will not go away, no matter what setting I use. I took it to the dealer and they said they had no idea what it was.

Wish there was something I could do.

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There is but it would be nice if VW would test this stuff and fix it themselves. In heavy rain, you use defrost, windows start fogging so you turn the fan on high.... should be good, right? heh. wrong. What happens is the airflow into the HVAC system exceeds what can sneak under the cowl cover, creating a negative pressure under the cowl. This pulls mist off wheels into the cowl area, which makes it's way into the hvac system. The same thing that happens in snow happens in rain, it's just less of a PITA. You can verify this by pulling out the pollen filter after driving in heavy rain, it will be wet. It's just a dumb design, made worse by trying to fix the snow problem.
The fix is to allow high pressure air from the cowl/windshield area to get to the hvac intake. My car is currently at the dealership where they are attempting to rectify this, if that doesn't prove fruitful, which I doubt it will as they are limited in what they can do, I'm going to install frogskin vents in the cowl. They are basically a fine plastic screen we use on snowmobiles to let cooling air in , and keep snow out of the hood. THey are available in 2" round and 3X4" rectangular panels. I figure a few of these on the cowl should allow enough air in to balance off the hvac requirements and prevent sucking snow and dust off the road through the drains.
It was just so much simpler with the pollen filter on top of the hvac intake, pity they screwed this up. Anyone know if an earlier passat or jetta hvac intake pollen filter holder will fit the newest passat hvac intake? This worked on A3 and 4 golfs to allow pollen filters on cars that never had them....

I agree, VW should fix these things themselves, but waiting for that to happen isn't worth it. I drilled out the perforation about 3-4 weeks ago. Blocked the holes in the fender and have actually not had any problems with the windows fogging any more. I don't think I will have to worry about the filter plugging none stop anymore either. It really is a quick simple fix, but one that you should never have to do on a 2013 vehicle. At least we can actually drive this thing in winter now.

Greg8558 said:

I agree, VW should fix these things themselves, but waiting for that to happen isn't worth it. I drilled out the perforation about 3-4 weeks ago. Blocked the holes in the fender and have actually not had any problems with the windows fogging any more. I don't think I will have to worry about the filter plugging none stop anymore either. It really is a quick simple fix, but one that you should never have to do on a 2013 vehicle. At least we can actually drive this thing in winter now.

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Did you fully block them? They are supposed to be water drains if I recall correctly? I'm tempted to put a sponge or something over them, trap some dirt but still get water out.

When you removed the cowl to drill the holes, is there actually anything that would prevent air flow from getting in, if you hadn't drilled the holes? I looked around and it doesn't seem like I actually need to drill holes, it looks like the area under the holes is already open to the engine compartment?

When I was sound proofing my car I made foam covers for the wheel drain holes to block sound to the windshield base.

These should work for those of you that have the unvented cowl and need to block snow or excessive moisture.

I bought some 3/8 rubber foam and cut them to the shape of the area, used a piece of cardboard to make my template.

At the top of the foam I added a small tab that I inserted into the cowl drain hole. By making it just slightly wider than the hole the friction holds it in place.

On the bottom I used a razor and cut/split the bottom of the foam about 3/8 deep. This allowed the wheel well liner to be inserted into the bottom of the foam and then I added a little form a gasket between them to help hold the bottom in place.

This has held up well for 8 months to include this winter in Mn. The cost is cheap to try, you can get a 2'x2' piece of foam for under $10. Just make sure it is not too flexible so that it stays in place.






Give it a try.

I'm not having any fogging issues that putting the car on defrost mode won't take care of( build date is 6/12 I believe )

drilling holes holes in the sealed up upper cowl is unnecessary.
the cut-out by the wheel well is in fact a drain, not an air intake.
Look at your cowl and see the huge gaps all around it where it meets the fender, specially where it gets closer to the firewall.
at least a 1/4 gap running alongside the fender area as well cutouts where it gets near the windshield.
If you add up all those gaps on top ( on both left and right side) you will have much more area (square inch) than the drain down by the wheel well.

^^^That's what I see on mine as well, although I've got the perforated cowl. Seems to have plenty of space to draw in air.

we have a 12 tdi Passat with the holes in the cowl.. But man do we have problems with snow getting sucked into the filter. It has been a nasty winter in Manitoba, with snow and cold. The forecast for Saturday the 1st of march is minus 50 with wind chill.
the snow has been very powdery and has been plugging up the air intake. We have two miles of gravel road that has approx. 4 foot banks on each side. everytime it is windy the road plugs up again. It will not blow much as something is getting plugged. We have a heated shop, and can dry things out.. But I a going to try to do something to slow this problem down..

See my post #75 for an easy and cheap fix.
Eric

my wife's car will also not completely defrost the windsheild no matter how you set it.. my 2000 golf will defrost the whole car front to back.. its somewhat disappointing.

I've always had to have the AC on, even will using the heat, to purge excess moisture from inside the cabin on my car.

Driver_found said:

I've always had to have the AC on, even will using the heat, to purge excess moisture from inside the cabin on my car.

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The AC doesn't turn on below freezing no matter what the button says