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Failed Thermostat Housing/Coolant Loss Info Gathering

stuntnuts

Ready to race!
Location
Maine
Two questions:

Was the solution to just replace the reservoir cap? Do you mind editing the post to reflect that solution? Thanks.
I haven't replaced the cap yet, so for me to say it was resolved would be dishonest. I don't do dealerships, and I haven't gotten to replacing the cap myself yet, but I know it's the issue.

My last Mk7, in the middle of the summer, you would actually see drips of coolant in that path. My current Mk7.5 just started doing it in the fall and I don't drive it in the winter, so I haven't addressed it. In the current car, I detect a slight odor on occasion, but not nearly as bad as my Mk7. I haven't seen any actual drips on this one yet. Just the dirt magnet vapor path.
 

stuntnuts

Ready to race!
Location
Maine
I thought the dirt buildup there was the result of some sealant or something during the manufacturing process... I've got a buttload of dirt and grit stuck on there but it doesn't rub or wash off, it's been there since the car was new. I also have a slowly dropping coolant level so I'll pay closer attention to that spot and see.
That line of "dirt buildup" was not there when I bought my vehicle, nor did it have the sweet aroma of coolant under the hood. It now has 6,500 miles on it.
 

theDoktor

Go Kart Newbie
Location
Buffalo, NY area
Car(s)
2017 GTI Sport
Good info @stuntnuts. Unfortunately my Mk. 7 gets driven in the winter plus it has Krown rustproofing. I'm going to have to clean off the excess rustproofing on the frame rails when spring finally arrives before I can isolate any possible overflow tank leakage. Right now, it's just a mess!
 

joesid

Ready to race!
The overflow tank IS pressurized, you fool. If you don't believe me, by all means, go for a nice long drive and park your car. Pop your hood and open your coolant overflow tank cap. You'll feel the pressure.....all 200+ degrees F burning all over your face. I'm serious, please, try it.

Anyways, back to the evidence of a coolant tank cap leak......Look for a trail of dirt that starts below the coolant bottle and travels down to the front of the vehicle, along the frame rail, like in this pic:

View attachment 165070

I've had two Mk7's and they both demonstrated this. THAT is your vapor leak. Some are worse than others. As you drive, the dampness attracts dirt to the path and when it dries, the dirt remains.

The overflow tank is SUPPOSED to release pressure IF the pressure gets too high, to protect the system. The problem is these caps are releasing pressure at too low of a pressure, intermittently.
Here’s mine, it goes in for service in a week. I’ve lost about 16oz since my last service (7,500k) and have no clue where it’s gone.
68A5BBDB-08E6-4C43-B12C-B6DACB642A6C.jpeg
 

dequardo

Drag Race Newbie
Location
Waukesha, WI
I haven't replaced the cap yet, so for me to say it was resolved would be dishonest. I don't do dealerships, and I haven't gotten to replacing the cap myself yet, but I know it's the issue.

My last Mk7, in the middle of the summer, you would actually see drips of coolant in that path. My current Mk7.5 just started doing it in the fall and I don't drive it in the winter, so I haven't addressed it. In the current car, I detect a slight odor on occasion, but not nearly as bad as my Mk7. I haven't seen any actual drips on this one yet. Just the dirt magnet vapor path.
Odd that you are so strident about this issue but have not yet addressed it yourself for the pittance of a new cap.
 

stuntnuts

Ready to race!
Location
Maine
Odd that you are so strident about this issue but have not yet addressed it yourself for the pittance of a new cap.
Odd, you say? My first car to exhibit this behavior was bought back by VW. My current car exhibiting this behavior just started doing it before I parked it for the winter. I avoid the dealership at all costs. I'm not going in there because my $20 coolant bottle cap is leaking to get it fixed under warranty. When I begin driving this car again in a month or two, I'll address it.

I'm stringent about it for OTHERS who have been strung along by VW dealership repair facilities, with their inability to recognize and solve simple car problems.
 
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joesid

Ready to race!
This might be a dumb question, but I'm under the assumption that when the cap fails, the leak doesn't come from cap area. Is this correct? Is coolant pushed through one of the hoses attached to it? and where to?

Weird fact: I used to have an old W124 Mercedes that had a hidden coolant recovery tank hidden behind the fender.
(Not my car)
2AA54444-F588-446B-BA4B-15D26837CEC0.jpeg
 
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stuntnuts

Ready to race!
Location
Maine
This might be a dumb question, but I'm under the assumption that when the cap fails, the leak doesn't come from cap area. Is this correct? Is coolant pushed through one of the hoses attached to it? and where to?
The seal on the cap releases pressure and diverts the relieved coolant (or just vapors) out of the bottle itself (behind the bottle), where you can't see. For lack of a better term, there's a "gutter" in there. In severe cases, it drips down onto the frame rail and that is what creates the "dirty path" in my original picture.

In this pic, the "gutter" is flipped up. That's where the escaping coolant/vapor goes and drips down onto the frame rail.

Borrowed pic:

 
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ReadTheBook

Drag Racing Champion
Location
Bay Area Smoke Hell
Car(s)
2017 Sport, 2019 AT
Anyone know the part # for the coolant reservoir cap?
5Q0 121 321

List price is $39:ROFLMAO:

73C0 121 321end cap
81H0 121 687 Ao-ring
Would replacing the o-ring suffice, I wonder? Is it the cap itself that is failing or is the o-ring just not good enough to withstand the temps/chemicals it is seeing?


I wonder if there is an o-ring made of Viton (higher temp rating and chemical resistance) that is a correct replacement.
 
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joesid

Ready to race!
So when I got home, I flipped the little shield over the “gutter” as depicted in post #206 and saw the characteristic white residue of dried coolant. I definitely recommend taking a peak.
 
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