Mystery Coolant Loss

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splinter-cell

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Hi,

I have been experiencing some mystery coolant loss of late in my A3 2.0 TDI (CUNA).

I had my water pump and timing belt replaced around 20k miles ago by an Audi dealership and have already had my EGR cooler replaced around 5K miles ago by an Audi specialist.

Recently I’ve been experiencing some mysterious coolant loss, with no obvious signs of a leak. The coolant level gradually drops over a week or so during normal driving, but becomes more apparent when I’ve taken the car on a motorway journey.

What’s more is that after a particular hard motorway journey, I’ll get a feint smell of warm coolant coming from the engine bay with again no obvious signs of a leak. The only trace I can see is some dried pink spots around the outside middle section of the expansion bottle.

Whenever I open the coolant tank, I can hear a hiss escaping, as well as some coolant rapidly returning to the bottle, which leads me to believe that the cap/bottle itself isn’t the issue.

What’s more, I can hear water loudly gurgling behind my dashboard, even though I have run the VCDS coolant bleed procedures as well as the traditional “run the car with the coolant cap off” but no matter what I try, I can still hear gurgling behind the dash.

Any ideas where the leak may be?

TIA
 
a3's are known for having problems with the interior heater matrix. Something to do with the silica packs that can burst and ending up blocking it.... that said try to back flush it... if that doesn't work you'll have to replace it. the part isn't expensive but the work to replace it is another story... best to DIY if you can and save yourself a 4 to 5 hour labour bill + the extra coolant they'll charge you at double the price.
 
northpole said:
a3's are known for having problems with the interior heater matrix. Something to do with the silica packs that can burst and ending up blocking it.... that said try to back flush it... if that doesn't work you'll have to replace it. the part isn't expensive but the work to replace it is another story... best to DIY if you can and save yourself a 4 to 5 hour labour bill + the extra coolant they'll charge you at double the price.
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Thanks for the reply. I have already replaced my heater matrix about a year or two ago. Did it myself and haven’t had any issues with the heating system. I get hot air coming out.
 
Pull the timing cover and check if the waterpump is leaking... but since that has recently been replaced I doubt it... also a simple replacement of the lid on the expansion tank could be needed as those things are supposes to let pressure out... they do fail over the years and they are cheap enough to replace.
 
northpole said:
Pull the timing cover and check if the waterpump is leaking... but since that has recently been replaced I doubt it... also a simple replacement of the lid on the expansion tank could be needed as those things are supposes to let pressure out... they do fail over the years and they are cheap enough to replace.
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Would the expansion cap be explain the air in the system?
 
It could be, but for the price of it just rule it out and replace it... pressure that doesn't escape turns into pressure pushed back on the cooling system this in return than pushes the coolant out of the overflow from the expansion tank.

But take note: any obstruction in the cooling system can trigger this and will have the coolant run out of the overflow from the expansion tank.
 
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Have you tried replacing the coolant tank cap? Had all the symptoms you describe and couldn't find any source of a leak. Replaced the cap and hey presto problem gone away. What's more when looking at the cap and its replacement couldn't find any differences.
 
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Knacker said:
Have you tried replacing the coolant tank cap? Had all the symptoms you describe and couldn't find any source of a leak. Replaced the cap and hey presto problem gone away. What's more when looking at the cap and its replacement couldn't find any differences.
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it has a build-in pressure escape... but it can fail over the years... if you read my other posts i actually said the same thing, replace the expansion tank cap (that is what you call the coolant tank but it's an expansion tank, both in function and name)
 
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I had previously replaced the coolant tank with a part from eBay about a year ago. I have ordered a new coolant tank from the dealer now and will also be replacing the heater matrix on the off chance it has become blocked. Thanks for your help. Will keep you updated
 
splinter-cell said:
I had previously replaced the coolant tank with a part from eBay about a year ago. I have ordered a new coolant tank from the dealer now and will also be replacing the heater matrix on the off chance it has become blocked. Thanks for your help. Will keep you updated
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If there is no crack in it... all you need to replace is the lid or cap or whatever you want to call the thing that closes it... the expansion (coolant) tank has an overflow that will start pushing out excessive pressure when the lid(cap) fails to release said pressure.

In other words don't waste money replacing stuff that isn't broken.
 
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Check the intake manifold aka intercooler maybe is leaking..

Sent from my VOG-L29 using Tapatalk
 
northpole said:
If there is no crack in it... all you need to replace is the lid or cap or whatever you want to call the thing that closes it... the expansion (coolant) tank has an overflow that will start pushing out excessive pressure when the lid(cap) fails to release said pressure.

In other words don't waste money replacing stuff that isn't broken.
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So I've replaced the whole expansion tank now with a new part from TPS and the coolant loss seems to have stabilised. I do notice that the coolant level does fluctuate from time to time but since I can also hear some water gushing behind the dashboard, I'm guessing there is an airlock in the system.

I guess the next step is to try and purge the air and I should be good to go. Thanks for your help!
 
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glad it is sorted... next time try the lid/cap first, as I'm almost certain the expansion bottle wasn't cracked.
 
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Any idea what is the best way to purge the air from the system in these cars. Tried the traditional "leave the cap open while the car runs" but that doesn't seem to make any difference.
 
Jack it up or park it so the front sits higher than the rear. Air will find it's way out quicker that way but leave the lid/cap off when doing this... that is so the air can leave the bottle immediately
 
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Ps and the car needs to be running that way so with the front up higher than the rear.
 
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