Coolant slow loss - suspecting the cap

[lhlan]

Hi guys

I have been "enjoying" an ever slow coolant loss.

Symptoms:
1. First, I saw coolant below the MIN line, so fill it up with distilled water to just above. (To be honest, I don't know where the coolant was from the beginning)

2. After one week, it coolant level came down 0.5cm and I told myself: SCREWED! Lets do something about it.

3. G12++ mixed with distilled water was in the order, and filled to MAX. I checked around with my mortal eyes and fingers and found no visual leak, but two things stand out: white mark on the rim of the cap, and some short of stains where a coolant line is connected.

4. Two weeks later, coolant level is now down 0.5 cm from max line. I have cleaned the white mark above, no new mark shows. Stains: still there, no new stains.

Everytime I took measurement of coolant and filled, I make sure engine is cold to be consistent.

Having said that, I have developed a habit of popping the hood up before start and after parking, just to "enjoy" the look of pink coolant in the tank.

Separate question, my blue cap seems to be different from yours, it doesn't have the two thin lines on top.

Under the cap, it says:1J0 121 321B and AUSTRIA.

And my tank was printed with O1S on top of it - weird!

No trouble with cabin heating, no over heating.

Should I go from the cap and replace it?IMG_20221109_093223.jpgIMG_20221015_185853.jpg

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[A4B8 2.0tfsi]

Probably the crack in the coolant pump, that sprays a fine mist while running. You need to see where your leek is, instead of buying and trying blind...

[lhlan]

Probably the crack in the coolant pump, that sprays a fine mist while running. You need to see where your leek is, instead of buying and trying blind...

I wish I know the root cause. Dealer has pressure tested it, and has replaced coolant pump 4 months ago.

Do you think the stains are normal? The only reason I suspect the cap was because that white mark - very small - that can be seen on the left of the cap - second picture.

[Kolbenringe]

check the water pump thread in my post history and put your phone in the water pump area and underneath to see if there's something there while you're searching for leaks anyhow.

[treginginco]

I just had the same issue on my 2013 allroad. A hairline crack in the reservoir bottle was the problem. My shop says it's a common problem. Cost me less than $100 to get it fixed.

[lhlan]

I just had the same issue on my 2013 allroad. A hairline crack in the reservoir bottle was the problem. My shop says it's a common problem. Cost me less than $100 to get it fixed.

Did you find any traits of fresh or dried coolant anywhere? And where was the crack?

I have done further research and found that my bottle is 8K0121403AC, that should go with cap 3B0121321. Somehow my existing cap is 1J0121321B - strangely. As a second owner of the car, I don't know which one is original.

[Smac770]

It's simple enough to pressure test both the tank and the cap, once you have the correct tools. Tank should hold 1 bar without loss. Cap should hold 1 bar without loss, and then open the relief at between 1.4 and 1.6 bar.

What is your actual engine? Three letter code on the sticker on the upper timing cover.

[lhlan]

It's simple enough to pressure test both the tank and the cap, once you have the correct tools. Tank should hold 1 bar without loss. Cap should hold 1 bar without loss, and then open the relief at between 1.4 and 1.6 bar.

What is your actual engine? Three letter code on the sticker on the upper timing cover.

Not with my car at the moment - sticking my butt to a computer since dawn.

But I know these four letters by heart: CJEB

[lhlan]

It's simple enough to pressure test both the tank and the cap, once you have the correct tools. Tank should hold 1 bar without loss. Cap should hold 1 bar without loss, and then open the relief at between 1.4 and 1.6 bar.

What is your actual engine? Three letter code on the sticker on the upper timing cover.

And I am developing this theory: cap is good, tank is good. Just that they are not meant to be with each other.

[Smac770]

Ok, so it's a Gen3 1.8T.

The tank is 8K0 121 403 AC (originally rev Q, later rev T, now rev AC) for all B8.5 4-cyl. And the cap is 3B0 121 321 for all B8.5. The cap comes with the o-ring (28.5x3.8), 1H0 121 687 A.

The odd thing in your pics is that cap doesn't look like mine at all. https://www.ecstuning.com/b-genuine-...cap/3b0121321/

[Kolbenringe]

That cap looks like a way earlier one which likely would physically fit, but I have no idea what the venting pressures would be.

I got an OE cap for like under 5 Euros a month or so ago.

https://www.avp-shop.de/audi-deckel-...ehlmittel.html

[lhlan]

That cap looks like a way earlier one which likely would physically fit, but I have no idea what the venting pressures would be.

I got an OE cap for like under 5 Euros a month or so ago.

https://www.avp-shop.de/audi-deckel-...ehlmittel.html

Oh my God! How often do you have to change cap?

[Kolbenringe]

Oh my God! How often do you have to change cap?

I don't know, TBH. I just recently changed my cap as preventative maintenance. I guess theoretically one could pressure test it, I guess one could also monitor that overflow drain channel under the cap on the tank (after cleaning), but that doesn't tell too much about pressure unless the coolant was just starting to boil out from expansion.

Also if it were me, and I might eventually preventatively change my tank, I would wish there was a larger one available, but I don't believe there is. But my situation is very different.

I would look into a new tank and cap, but before you do all that work, really go around and try and look for leaks elsewhere and especially at the pump and thermostat housing. Then you can knock everything out all at once.

Also I double checked and my cap was 3.78 from Amazon. I simply had it in my basket for a while, then I saw the price went down, and I sniped it. It's now 4.99. I ordered a Febi and the exact OE cap was delivered.

[lhlan]

I don't know, TBH. I just recently changed my cap as preventative maintenance. I guess theoretically one could pressure test it, I guess one could also monitor that overflow drain channel under the cap on the tank (after cleaning), but that doesn't tell too much about pressure unless the coolant was just starting to boil out from expansion.

Also if it were me, and I might eventually preventatively change my tank, I would wish there was a larger one available, but I don't believe there is. But my situation is very different.

I would look into a new tank and cap, but before you do all that work, really go around and try and look for leaks elsewhere and especially at the pump and thermostat housing. Then you can knock everything out all at once.

Also I double checked and my cap was 3.78 from Amazon. I simply had it in my basket for a while, then I saw the price went down, and I sniped it. It's now 4.99. I ordered a Febi and the exact OE cap was delivered.

Excellent advice. I will follow yours and see the pump first.

Do I observe it when engine is warm to temperature and is running? Should the heater be on then?

I suppose I don't have to get under the car it for this inspection.

Thanks - and by the way - here in Australia the OE cap is less affordable! - around an equivalent of EUR 15. Volkswagen's genuine will set me back EUR 25.

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[RPMtech147]

I wish I know the root cause. Dealer has pressure tested it, and has replaced coolant pump 4 months ago.

Do you think the stains are normal? The only reason I suspect the cap was because that white mark - very small - that can be seen on the left of the cap - second picture.

Pressure test again, throw a cap at it. Wouldn't be the first time a new water pump leaked or was installed wrong.

[lhlan]

Pressure test again, throw a cap at it. Wouldn't be the first time a new water pump leaked or was installed wrong.

Cool, will do and will throw a cap nonetheless - just so as it does not look odd from the rest :-)

Under the hood aesthetics are of paramount important.

A4 B8.5 1.8 TFSI - proudly smooth Multitronic

[Kolbenringe]

Excellent advice. I will follow yours and see the pump first.

Do I observe it when engine is warm to temperature and is running? Should the heater be on then?

I suppose I don't have to get under the car it for this inspection.

Thanks - and by the way - here in Australia the OE cap is less affordable! - around an equivalent of EUR 15. Volkswagen's genuine will set me back EUR 25.

Sent from my Mi 9 SE using Audizine Forum mobile app

Check my other thread on replacing a water pump. There are pics in there. You basically shove a phone with the light on or use a strong flashlight in the area of the driver's side of the block, towards the bellhousing, and down. To check if the housing is leaking, you'll find a a small gap through wires and hoses towards the front and top on the driver's side and you'll be able to see a small shelf on the block there where coolant would leak to.

If the water pump and/or housing, there's a gap where the coolant tends to drain down to where the crank position sensor is. That's what you're looking for in the first scenario near the bellhousing. If the housing, the coolant will end up there as well, but more coolant will be forward, like on the crankcase ventilation housing. If coolant is externally leaking, there should always be a smell. I had one, even after installing my second to last pump. It's gonee after two years now that I finally have a good pump, housing, and housing to block seal in place.

I also note that people overfill their expansion tanks and wrote about it here: most cars I've seen, and including my Audi dealer, would fill the tank when cold to the "max" mark. This is not good. I usually fill my tank to 60% between min and max and when hot in the winter, the level goes to just barely under max and in summer or during hard driving, to the max. The expansion tank here is designed to do a few things, from making sure the tops of the radiators and blocks are free from air to allowing expansion so components don't blow. Not recommended, but if you have your system filled to max, when it's hot and you do the slowly remove the cap trick to avoid coolant blowing all over you, you'll see all the hoses shrink significantly more than if you were to just fill between the marks. People used to fill to max n the Audi 100/200 days and their tanks would end up splitting.

Also does anyone know if there are any variants of the B8 or 8.5 (or even 9) that have a larger expansion tank?

[lhlan]

Ok, so it's a Gen3 1.8T.

The tank is 8K0 121 403 AC (originally rev Q, later rev T, now rev AC) for all B8.5 4-cyl. And the cap is 3B0 121 321 for all B8.5. The cap comes with the o-ring (28.5x3.8), 1H0 121 687 A.

The odd thing in your pics is that cap doesn't look like mine at all. https://www.ecstuning.com/b-genuine-...cap/3b0121321/

Update at 1 March 2023

Cap replaced end of November, in the first few weeks, no change in coolant was registered. So I was happy and totally forgot about it - or I tried to not think about it.

Fast forward 3 months later: car was due for major maintenance (120k km). Mechanics advised: coolant is at low level! So obviously I have been loosing coolant, just at a lot slower pace than before! (used to be 0.5cm per week).

Pressure test requested, culprit found, and picture taken: a tiny leak in radiator. See picture.RadiatorLeak.jpg

The guy said: with this tiny leak, you can still drive for months, just keep an eye on the level. He maybe right, the level of coolant loss is much less than typical oil consumption! :-)

I am not happy now. What should I do? Can this be repaired or have to be replaced?

Anything else around the radiator or the cooling system should be replaced as well when I am at it?

[lhlan]

Story continues:

Radiator replaced, and with the new cap - Febi, I thought My A4 would be great again.

Unfortunately, it kept losing coolant, albeit at a much lower rate: MAX to MIN in 4000 km.

Inspected the new Febi cap because I needed to add coolant anyway, I found something odd about it: it can be jiggled back and forward (something not tight here), it left a non-continuous black mark inside the tank mount (aha - the O-ring rubs unevenly against the mount), and most worryingly: the inside of its blue housing is wet! It must have leaked!

Here is a picture of my busted Febi cap. I marked red crosses where heavy water droplets were found.

Attachment 317058

And the marks its O-ring left on the inside of my expansion tank.

Attachment 317059

I compared that new Febi cap to my original one, and found two stand-out differences:

1. The Febi's inner valve travels much less.
2. The Febi's O-ring cross section is shaped like a rectangular / square while the original cross section is round.

So my previous old cap went in place of the Febi. 500 kms later coolant has not dropped one bit.

I complained to the seller and they would send me a replacement.

A little disappointed in Febi in this department, but I believe in second chance.