windshield inside fogging

Only occurs after heavy rain and more so in warm weather. Ae 10k service had dealership snip off cowl drain duckbills and sunroof and a/c condensate drains as well. My b5.5 cowl drains were backing up until I deleted the duckbills. Sitting outside for 2 years my 2015 had accumulated silt in the drans

You know, the NMS seems to have been poorly designed in the HVAC department, specifically the ability to get ample fresh air into the cabin. I know it has undergone a few revisions, and there are TSBs about updated parts, etc.

I have noticed this on the few that I have driven in damp, cooler (but not freezing) weather, too.

Oddly enough, my 1982 Audi 5000CS was the exact same way. It seemed even if it was 50 F out, you HAD to have the compressor running to keep the windows from fogging up.

Not sure what to do about it beyond making sure the newest pieces are installed.

Is the A/C working? Fog inside cabin is humidity that then condenses on the cold windshield. A/C (tempered up) should dry it on defrost setting.

We battle with this issue all the time in south Texas where it's humid all the time (we live in a swamp so to say).

Fixing sunroof drains should have not much to do with solving the humidity problem, but should help if you fix the cowl drain as cowl accumulation of water can get into the pollen filter making it wet all the time,which would allow humid air to enter the cabin.

Any of this make sense?

Cheers,

Tony

Sounds like a leaking heater core.

whitedog said:

Sounds like a leaking heater core.

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I know that tell tale sweet smell! TS didn't mention that though.

no glycol, just massive fogging 3/4 up inside w/shield. Just got it back today with the deleted cowl drain d/bills. That should end my problem. It only occurred during very heavy rain and never with just high humidity.

ours did it once, but the wife didn't have the ac button pushed- she had water dropplets falling onto the dash

Is your carpet wet? Check my Post with regard to leaking firewall seams.

Could not get the humidity out of the cabin. Go figure, I was swimming in a small pond.

ive had more of a greasy film then fog on the inside, a good elbow rub and a glass cleaner and it really helps out.

I've also noticed the same on the mk7, I mark it up to outgassing of interior bits and my vile breath greasing up the windshield.

oilhammer said:

You know, the NMS seems to have been poorly designed in the HVAC department, specifically the ability to get ample fresh air into the cabin. I know it has undergone a few revisions, and there are TSBs about updated parts, etc.
I have noticed this on the few that I have driven in damp, cooler (but not freezing) weather, too.
Oddly enough, my 1982 Audi 5000CS was the exact same way. It seemed even if it was 50 F out, you HAD to have the compressor running to keep the windows from fogging up.
Not sure what to do about it beyond making sure the newest pieces are installed.

Click to expand...

I've found the system to perform very well both in the heating and cooling departments (until the heater cores clog up, that is).

The one problem is its tendency to fog the interior of the windows in automatic mode when it's raining, especially in cooler weather. This happens because the system is not naturally aggressive with its dehumidification strategy in order to maximize fuel economy.

If you want maximum dehumidification, you have to turn on the windshield defrost mode in order to get maximum compressor output combined with heat. It defaults to a strong fan speed, but you can turn it down manually. Set the temperature for whatever you like and enjoy a comfortable fog-free car.

The defrost/floor split works okay, but not nearly as well as the maximum windshield defrost mode.

If Volkswagen had added a humidity sensor to the top of the windshield like the Phaeton has, this wouldn't be an issue.

The rental (2.5L of course) NMS Passat I used for a snowy weekend had adequate defogging/defrosting, but it was not as good as some other cars I have experienced. It NEEDED the A/C button "on" the whole time, otherwise the interior windows would quickly fog back over. And it needed the setting kept on defrost mode most of the time. Something my B5 Passats do not need generally. Once the windows are clear, they remain so even after the dehumidification device (compressor) has been switched off.

Not a big deal, but I have had other customers complain of similar things. Which is certainly why VAG has at least made some attempt to improve the system over the NMS' tenure.

I would not crucify the platform over this, though. Like I said, my top-of-the-line Audi was the same way.

my Passat hasn't had any fogging since the duckbills were snipped off the cowl drain hoses. Apparently in really heavy rain there may have been water laying in the cowl tray. On my B5.5 I deleted the whole hoses and ended the clogged drain issue.

Is there a DIY for the duck bill surgery?

Also the HVAC throws pretty poor heat at your feet.

740GLE said:

Is there a DIY for the duck bill surgery?

Also the HVAC throws pretty poor heat at your feet.

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Ha! That was the other thing I noticed. Forgot about that!

Yeah the floor heat sucks.

Well, not fixed on 2017 MY either... was the first issue I noticed on rainy days -- even when driving solo.

This issue has me worried about performance in winter weather (more so when floor mats & shoes damp), however this morning's commute was 15°F with no problems -- despite using recirculate w/o AC the whole time.

I will investigate the duck bills... maybe test next Spring by inserting drinking straws as a temporary hack.

MyAvocation said:

Well, not fixed on 2017 MY either... was the first issue I noticed on rainy days -- even when driving solo.

This issue has me worried about performance in winter weather (more so when floor mats & shoes damp), however this morning's commute was 15°F with no problems -- despite using recirculate w/o AC the whole time.

I will investigate the duck bills... maybe test next Spring by inserting drinking straws as a temporary hack.

Click to expand...

I'm pretty sure the "duck bills" were redesigned at some point mid-way through the NMS lifecycle. IIRC it was only a problem on 2012s and 2013s, but I could be remembering incorrectly.

Recirculate at 15F? I don't think it will even do recirculate at that temperature, although the light might still come on. A/C definitely won't come on at that low of a temperature.

Since de-ducked no further problem. Yes, the 2015's have them.

When I did my heater core replacement I left the driver side floor duct and kick panel off and had much better heat results just letting the heat blow from the large hole in the hvac assembly. It also seemed to eliminate the cold air sloshing many speak of. I always had ridiculous fogging of the windshield and max defrost was the only way to start a drive in the spring and fall

thundershorts said:

Since de-ducked no further problem. Yes, the 2015's have them.

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Thanks for clarifying.