Help! Misterious coolant loss - S6 with 50k miles

[danmm7]

Hello All,

Over the last few days, I've noticed that my coolant level has dropped under the "min" level in my coolant tank on a few occasions. I keep adding and it still drops.
** No visible leaks or strong coolant smell. **

Never had an issue before. The coolant level warning light never came on, I just noticed the level was under "min" last week, so i started to monitor and top it off.

The car is very well maintained. 50k miles. It runs DS1 tune with stock turbos. I also installed a larger heat exchanger 3 months ago.

2 weeks ago we took a 250-mile trip to the mountains (Lake Tahoe) and there were no issues. Once we got there, I drove around town, lots of snow, ice and slush on the road, but nothing really crazy that should impact my cooling system.


Last week i thought i smelled a very faint coolant odor from the front grill area. Opened the hood and saw my coolant under the "Min" mark. Visually, i did not see any leaks or any sources of a strong antifreeze smell from the top. No white stains.
I topped off the level to "max". Went to town (still in the mountains), car drove normally...it was about 20-25-mile trip. Came back home and the coolant was at the "min" mark!!! :( Still no leaks or obvious smell from anywhere.

That got me worried since next week i need to drive back home, and that's a 250 miles drive.
I topped the coolant to Max, and drove back to town to buy some ramps and tools so I can get under the car and remove the covers and inspect for leaks.
This time, after the 20 miles trip to town .... my coolant DID NOT drop one bit. Like I never lost coolant before.

I went under the car, removed the covers and i did not see anything obvious. No leaks, no puddles, no white stains, nothing really obvious from below the engine area. I only noticed one single drop of coolant on the thermostat housing, but i am not sure if it came from it, or it was from a small spill above when I was topping off the coolant tank. The thermostat housing itself was clean and nothing around it had coolant stains or smell. The plastic covers that go under the car were also dry/clean, with no visible coolant stains on them or smell.

** Keep in mind, before I went under the car to inspect, I've been topping off 2-3 times, and easily have added close to a 1/2 quart of coolant total. If there was a leak, that 1/2 quart of coolant had to be visible somewhere under or around the engine. But it's not. It's All is clean.

Since the level was stable and I found no leaks under the car, I assumed that maybe I had an air bubble somewhere in the system and it got evacuated during the long drive to the mountains.

Well, this morning, after the car has been sitting parked for 48 hrs, (we got snowed in), i cleaned all the snow, looked under the hood, and the coolant level was where i left it 2 days ago (good!!) ... then i started the engine to warm up the car, let it run for 15 min or so.... checked under the hood and the coolant was under the "max" again. It dropped, just by keeping the engine at idle!! (bad!!) So coolant is still going somewhere! :(


So where is my coolant going if no visible leaks?!?! How serious could this be.... and more importantly, would I be able to make my drive back home next week, with occasional topping off, or am i playing with fire? Closest dealership to my location is 1.5hrs away. Not many German car shops here either.


Thanks for any ideas!!

[qcrazy]

The most common leak point is the connectors to the IC on top of the engine. Really common. The coolant goes into the vee and slowly burns off. It is really hard to see the leaks, check these connections really closely. Those clamps are known to leak.

Another common point, since you changed the HX, is aftermarket HX connections. Often those clamps are not tightened enough. When those leak the coolant drops on the inside of the bumper and then is "aerosolized" out under the car. Only puddles when it gets really bad. The drop on the t-stat housing is in this area. Easiest way to check these leaks is looking through the grille, hopefully it is visible there on your car.

[Billal08]

I’m having a similar problem with my S6 c7.5

I’m just over 50k miles.

The coolant loss seems to get worse in the cold weather. I’ve pressure tested with no clear outcomes, audi main dealer has also had a look and can’t find anything.

It’s sporadic I am a bit lost as to what I should do too, so will deffo be grateful for any replies on this!


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[Audibellybutton]

Exactly what Q said, but check that thermostat housing. They are known to leak and cause a small coolant loss that you will not see without looking at the thermostat. If it looks crusty then its leaking and you can change the housing, or just the gasket. If you don't have a code for the thermostat you likely don't need to change the whole thermostat

[TexasDfwS4]

The most common leak point is the connectors to the IC on top of the engine. Really common. The coolant goes into the vee and slowly burns off. It is really hard to see the leaks, check these connections really closely. Those clamps are known to leak.

Another common point, since you changed the HX, is aftermarket HX connections. Often those clamps are not tightened enough. When those leak the coolant drops on the inside of the bumper and then is "aerosolized" out under the car. Only puddles when it gets really bad. The drop on the t-stat housing is in this area. Easiest way to check these leaks is looking through the grille, hopefully it is visible there on your car.

If I was having this problem I would lessen to this wisdom.

[Lilrckt]

I’m having a similar problem with my S6 c7.5

I’m just over 50k miles.

The coolant loss seems to get worse in the cold weather. I’ve pressure tested with no clear outcomes, audi main dealer has also had a look and can’t find anything.

It’s sporadic I am a bit lost as to what I should do too, so will deffo be grateful for any replies on this!


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Same thing here had some loss last winter, took it in pressure test etc, nothing. Then just fine all summer, no noticing coolant loss again... What to do?

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[Jamos]

Yup same. Had tstat replaced, no visible leaks ANYWHERE, but always smelled coolant in the summer. All hoses are clean, no building anywhere. Like... eugh Audi! My RS5 always smelled the same, and it turned out it was a hairline crack on the return coolant line to the reservoir. Not visible on the S7, but there is certainly a loss after spirited driving - so I generally just put around town these days, what a total waste.

[Sanjman]

Following
Same issue here tech can't find the leak

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[ichi d]

having the same issues too.

[lhlan]

Hello All,

Over the last few days, I've noticed that my coolant level has dropped under the "min" level in my coolant tank on a few occasions. I keep adding and it still drops.
** No visible leaks or strong coolant smell. **

Never had an issue before. The coolant level warning light never came on, I just noticed the level was under "min" last week, so i started to monitor and top it off.

The car is very well maintained. 50k miles. It runs DS1 tune with stock turbos. I also installed a larger heat exchanger 3 months ago.

2 weeks ago we took a 250-mile trip to the mountains (Lake Tahoe) and there were no issues. Once we got there, I drove around town, lots of snow, ice and slush on the road, but nothing really crazy that should impact my cooling system.


Last week i thought i smelled a very faint coolant odor from the front grill area. Opened the hood and saw my coolant under the "Min" mark. Visually, i did not see any leaks or any sources of a strong antifreeze smell from the top. No white stains.
I topped off the level to "max". Went to town (still in the mountains), car drove normally...it was about 20-25-mile trip. Came back home and the coolant was at the "min" mark!!! :( Still no leaks or obvious smell from anywhere.

That got me worried since next week i need to drive back home, and that's a 250 miles drive.
I topped the coolant to Max, and drove back to town to buy some ramps and tools so I can get under the car and remove the covers and inspect for leaks.
This time, after the 20 miles trip to town .... my coolant DID NOT drop one bit. Like I never lost coolant before.

I went under the car, removed the covers and i did not see anything obvious. No leaks, no puddles, no white stains, nothing really obvious from below the engine area. I only noticed one single drop of coolant on the thermostat housing, but i am not sure if it came from it, or it was from a small spill above when I was topping off the coolant tank. The thermostat housing itself was clean and nothing around it had coolant stains or smell. The plastic covers that go under the car were also dry/clean, with no visible coolant stains on them or smell.

** Keep in mind, before I went under the car to inspect, I've been topping off 2-3 times, and easily have added close to a 1/2 quart of coolant total. If there was a leak, that 1/2 quart of coolant had to be visible somewhere under or around the engine. But it's not. It's All is clean.

Since the level was stable and I found no leaks under the car, I assumed that maybe I had an air bubble somewhere in the system and it got evacuated during the long drive to the mountains.

Well, this morning, after the car has been sitting parked for 48 hrs, (we got snowed in), i cleaned all the snow, looked under the hood, and the coolant level was where i left it 2 days ago (good!!) ... then i started the engine to warm up the car, let it run for 15 min or so.... checked under the hood and the coolant was under the "max" again. It dropped, just by keeping the engine at idle!! (bad!!) So coolant is still going somewhere! :(


So where is my coolant going if no visible leaks?!?! How serious could this be.... and more importantly, would I be able to make my drive back home next week, with occasional topping off, or am i playing with fire? Closest dealership to my location is 1.5hrs away. Not many German car shops here either.


Thanks for any ideas!!

Are you able to sort out this coolant issue?

[omega_jeff]

subscribed

[snohman]

I had a low coolant issue as well - although it turned out to be much simpler.

Initial warning showed low coolant so simply added in more below the max line. Couldn’t find any sort of visible leak. After about a month, the light came back on. Again refilled to the top as it had visibly fallen below the minimum line. This time, I saw a slow drip from the bottom of my coolant tank which had cracked.

We had unusually colder weather in SoCal so not sure if that contributed at all (seems doubtful) but the crack only seemed to have gotten larger making the leak more noticeable later on. Was driving me crazy but was a simple fix of replacing the tank and one of the hoses.

[rlny]

It’s the cold weather. For some reason the water evaporates in really cold temps.


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[Lilrckt]

The mysterious puddle showed up tonight in 20 degree f temps, just like it did last year. Puddle is underneath the front of the engine on the driver's side. Haven't torn anything apart, but maybe that helps with a common leak point in the cold.

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[Lilrckt]

Looks like leaking out of the bottom of the housing. Bottom two bolts are tight. Guess it's off to the shop to get whatever this is replaced 20230225_113739.jpg

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[goldenzrule]

The most common leak point is the connectors to the IC on top of the engine. Really common. The coolant goes into the vee and slowly burns off. It is really hard to see the leaks, check these connections really closely. Those clamps are known to leak.

Another common point, since you changed the HX, is aftermarket HX connections. Often those clamps are not tightened enough. When those leak the coolant drops on the inside of the bumper and then is "aerosolized" out under the car. Only puddles when it gets really bad. The drop on the t-stat housing is in this area. Easiest way to check these leaks is looking through the grille, hopefully it is visible there on your car.

I'm trying to figure out exactly where you mean. Any chance someone has a picture they can post to show where to check?

[qcrazy]

Check where circled. They barely leak, you can't really see it at the hose. You really have to look below and look for traces of coolant. You can see my upgraded clamps.
20230226_185111.jpg

If you haven't upgraded your HX, you don't have to worry much about the second item I discussed. If you did, you will see coolant on the item pictured two posts above (black thermostat housing). You'll get a few drops where shown, but the whole thing will be wet due to being aerosolized.

The picture above is a bad o-ring on the thermostat housing. You can see how it is wet all along the interface, and then accumulates at the bolt. That is probably the third most likely leak point. And a bigger pain to fix.

[danmm7]

Check where circled. They barely leak, you can't really see it at the hose. You really have to look below and look for traces of coolant. You can see my upgraded clamps.
20230226_185111.jpg

If you haven't upgraded your HX, you don't have to worry much about the second item I discussed. If you did, you will see coolant on the item pictured two posts above (black thermostat housing). You'll get a few drops where shown, but the whole thing will be wet due to being aerosolized.

.

I am the OP. Still, no progress on this issue, but I am happy to report that the loss of coolant has NOT increased. I haven't added any coolant for the last 800 miles but now i can see a bit of a level drop in the tank.

I pulled out the IC since many people said that I may have a leak inside the IC box. No leak. All dry... and ZERO coolant smell in or around the IC assembly. It can't be there.

I can smell a faint coolant odor from the driver's side every time the car is warm but i have no idea where it's coming from.

One thing i realized is that the coolant tank itself must have a valve/opening somewhere on the bottom to release coolant when overfilled. How do i know? Well, my coolant tank was a bit overfilled, car was hot, and i decided to open the coolant tank (using gloves). It was pressurized but as soon as i unscrewed the blue cap, i saw how some coolant was released UNDER the tank. It almost sprayed a bit, to relieve the vacuum/pressure. It was weird, but i am 99% i saw it spray coolant from under the tank itself. There must be some sort of valve there. Anyone seen this?

I wonder if this valve may be acting up under certain conditions/temps ... and allowing a bit of coolant to leak out of the plastic tank?

[qcrazy]

There is no valve on the tank. It has an overflow, that will relieve when it is overfilled/pressured. But it has to flow up and out. Nothing like a valve on the bottom like you are referencing. You should inspect the tank and make sure there are no cracks in it.

If you smell it on the drivers side it is probably going to be best to pull off the undertray and take a look. The thermostat housing pictured above is on the drivers side.

Also, double check your hx clamps and make sure they haven't loosened up on you. You should be able to see the hx clamps through the grille.

[theskipper]

Just went through this with my 2016 S6. Found a little bit of coolant on the thermostat housing (lilrckt's picture) running back across the belly pan. Still under extended warranty, so packed it off to the dealer. They diagnosed an "internal leak" in the coolant tank and, at least according to the invoice, replaced only that. I'm skeptical, but not seeing any further leaking underneath. Cleaned all that area thoroughly to keep an eye on it. There is also a TSB from 2018 https://static.nhtsa.gov/odi/tsbs/20...38342-9999.pdf that provides a good overview of leak diagnostics on the 4.0 TT. According to that, a very small leak from the water pump is actually a feature to keep the seals lubricated. Hope this helps.

[Audibellybutton]

Just went through this with my 2016 S6. Found a little bit of coolant on the thermostat housing (lilrckt's picture) running back across the belly pan. Still under extended warranty, so packed it off to the dealer. They diagnosed an "internal leak" in the coolant tank and, at least according to the invoice, replaced only that. I'm skeptical, but not seeing any further leaking underneath. Cleaned all that area thoroughly to keep an eye on it. There is also a TSB from 2018 https://static.nhtsa.gov/odi/tsbs/20...38342-9999.pdf that provides a good overview of leak diagnostics on the 4.0 TT. According to that, a very small leak from the water pump is actually a feature to keep the seals lubricated. Hope this helps.

Oh wow nice, thanks for that info. I did not know that.

In my case I did have a code for thermostat, so I was sure that was the issue. But its good to know for other people, seeing a small amount of coolant does not mean the thermostat is leaking

[Lilrckt]

Oh wow nice, thanks for that info. I did not know that.

In my case I did have a code for thermostat, so I was sure that was the issue. But its good to know for other people, seeing a small amount of coolant does not mean the thermostat is leaking

Mine was always a tiny amount, but in 25f weather it was about 1/6th gallon overnight

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