Coolant leak - but no leak when pressure tested

[sirmium]

Hello, i just bought 2013 S6 with 40k miles from forum member. Car is in awesome shape, great seller to deal with. Drove it from TX to MI (1300 miles) without a hiccup, super happy with the car. I took car in to a dealer to replace few cosmetic things, peeling start stop button, grab handle, trunk liner handle. I also had them check side assist as it threw error during the drive, they updated side assist software under TSB 2034000/4.
[HISTORY] Dealer routinely found coolant leak and replaced turbo coolant pipes under TSB 2036167/2 . After the service i drove car 2-3 times no longer than 15 mins each drive, no issues at all. I also knew full service history of the car and seems that in 2013 dealer replaced O rings on these pipes but not the pipes.

[ISSUE DESCRIPTION]: On Friday i take 40 minute drive, no problems. Car was parked for about 2 hours, before i started driving back home. 5 minute into the drive on the highway, i get warning on the dash "Coolant temperature too high, park and let engine run". I get off the highway on next exit, pull up into not so nice neighborhood, open the hood, and see there is coolant on parts of the engine cover, and some steam from the overflow tank. Didn't feel safe in the area(some shady people started approaching, asking for money), so i drove another 2 miles to safer spot and waited for Audi Roadside assistance. There was still fluid in the overflow tank when i drove. Coolant was boiling in the overflow tank after i pulled over. After inspecting car carefully, there was coolant everywhere, undercarriage, wheels, engine cover....

Took Audi roadside assistance 40mins to arrive. Towed to local dealer, 10 miles away. They acknowledge coolant everywhere, did pressure test same day and surprise NO LEAK. They dont know how coolant got all over the engine bay. They are still investigating, but here are few theories so far:
Dealer theory no. 1 - Maybe when it engine gets hot some seals give in and there is a leak. Or he mentioned something about auxiliary pump kicking in, but i didnt follow all the way, but anyways, they tried to heat it up and no problems found. [DISPROVEN]
Dealer theory no. 2 - Maybe there was large air bubble(gap) in the radiator end after it finally burped, caused to spill coolant through pressure release valve. Just a theory, cant be proven, since air bubble is gone. Seems strange to me.

SA said there is 2 more things to test tomorrow, it's just taking long because they were heating up the engine and waiting for it to cool down, that takes time.

My theory no1. When performing TSB two weeks ago tech didnt clamp something and after 40 minute drive and more heating up hose flew off. Possible?

ANY IDEAS? I'm kind of mind boggled. Sorry for the long writeup, but I feel this strange issue best described in details for clarity. And pictures.

[sirmium]

Dealer cant replicate the issue, so i have no other choice but to drive it to failure. Thermostat was replaced in 02/16 and they say it's very unlikely for it to fail again, their theory is still that there was air gap in the radiator that caused the failure.

[agent47]

Did you try talking to Ryan? He is one of the best guys I have dealt with myself. Maybe try asking him what could have caused this and suggest that to the dealer?

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[sirmium]

Did you try talking to Ryan? He is one of the best guys I have dealt with myself. Maybe try asking him what could have caused this and suggest that to the dealer?

Sent from my SAMSUNG-SM-N920A using Tapatalk

I did, he was very helpful. He thought it's very likely that it could be thermostat again, even though it was replaced in February under warranty. Dealer doesn't have any codes for thermostat fault though, but Ryan said thermostat can fail without a code. I tried to talk to a dealer to replace thermostat anyway, but AoA will not approve warranty replacement without code or replicated failure. Manager got involved last night, they should call me by lunch today to let me know if AoA changed their mind and approved replacement or if I'll just pickup the car. And I understand dealers point of view also, it's sucky situation.

[agent47]

I did, he was very helpful. He thought it's very likely that it could be thermostat again, even though it was replaced in February under warranty. Dealer doesn't have any codes for thermostat fault though, but Ryan said thermostat can fail without a code. I tried to talk to a dealer to replace thermostat anyway, but AoA will not approve warranty replacement without code or replicated failure. Manager got involved last night, they should call me by lunch today to let me know if AoA changed their mind and approved replacement or if I'll just pickup the car.

He is actually right. Thermostat will fail without a code a lot of times. Technically the dealership needs to heat the car up on a long drive rather than a 10min stint. Hope the gremlins get sorted out for you man.

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[MLee]

same thing happened to my car, thermostat failed and coolant spilled out everywhere. they replaced the tstat and it was fine for 6 months or so. then one day coolant exploded out everywhere again, towed to dealer and they said the same thing. they didn't know where it was coming from, they did some kind of software update and told me to take the car without really fixing anything. I had a 2013, had to get the fuel line replaced twice and got stranded twice with an overheating engine because coolant spilled out everywhere. I ended up selling the car to CarMax because I was sick of being stranded on the side of the road.

[wuk]

2013 A6 here....started up the car after being parked for 5 hours and got the "Turn off Ignition. Check Coolant Level" error. Coolant level was below the Min level. 2 days later took car into the dealership asking them to take a look. They reported that there were some bad seals. In all, the receipt shows they replaced the thermostat, multiple gaskets and washers, and did something with a compressor). Got car back that day, and 1 week later everything is seemingly ok.

[sirmium]

to report back, dealer never found issue. They drove for ~50miles, no issue. Tested pressure, bled coolant. They offered to drive for 2x1.5hr trips, managers commute home, which i politely declined and took the car back.

Since back i did 200 mile road trip the following day, another 150 mile round trip week later, and daily commuting to work, no issues. 600+miles or so on the car since, no issues at all.

I'm taking it for an oil change in a few weeks and i'll ask them to check pressure again and inspect everything for leaks. I have another 8 months and 8k miles on the factory warranty and i hope i can get everything(if anything) resolved before that expires.