Coolant Leaking - Head Scratcher

[therzza]

Gents - trying to diagnose a frustrating coolant leak. Mild day, 60*, driving to drop off my kid, get a “low coolant shut off engine now” warning…odd, no puddle under the car, but sure enough the reservoir is dry. Fill it up, no issues for a few weeks. Open the garage one random Saturday and there’s a river of coolant flowing from the car (it’s not a daily). Get it up on stands, look for signs of a leak…nothing. Checked the usually suspect - thermostat housing, etc. No sign of a leak. Pressure tested the system at 12-13psi, it held overnight, no leaks. I replaced the lines with TGK braided, replaced the reservoir, thought that might’ve fixed it…took it out on a hot day (90*) this week, no leaks. Following morning, it’s all over the garage floor again. Started it to air up the suspension so I could get a drain pan underneath and the leak stopped. Hasn’t leaked since (it’s just been sitting).

No idea where to look now. When it leaks it’s dripping from somewhere above the sway bar on the driver’s side, but when it’s up on stands I can’t see a leak anywhere. Probably gonna take it to an indi shop, but would love to fix this myself. Car is a 2016 S6 with about 44k miles on it.

Any ideas from folks who chased leaking coolant issues? Thanks for reading.

[Burkeomatic]

Gents - trying to diagnose a frustrating coolant leak. Mild day, 60*, driving to drop off my kid, get a “low coolant shut off engine now” warning…odd, no puddle under the car, but sure enough the reservoir is dry. Fill it up, no issues for a few weeks. Open the garage one random Saturday and there’s a river of coolant flowing from the car (it’s not a daily). Get it up on stands, look for signs of a leak…nothing. Checked the usually suspect - thermostat housing, etc. No sign of a leak. Pressure tested the system at 12-13psi, it held overnight, no leaks. I replaced the lines with TGK braided, replaced the reservoir, thought that might’ve fixed it…took it out on a hot day (90*) this week, no leaks. Following morning, it’s all over the garage floor again. Started it to air up the suspension so I could get a drain pan underneath and the leak stopped. Hasn’t leaked since (it’s just been sitting).

No idea where to look now. When it leaks it’s dripping from somewhere above the sway bar on the driver’s side, but when it’s up on stands I can’t see a leak anywhere. Probably gonna take it to an indi shop, but would love to fix this myself. Car is a 2016 S6 with about 44k miles on it.

Any ideas from folks who chased leaking coolant issues? Thanks for reading.

Probably the cursed coolant seals above the T-Stat. They only leak when it's cool and the car has been off for a while. When it's hot out or the car is warm, it won't leak. The colder it gets, the worse it leaks.

[therzza]

As in, that dumb plastic coolant pipe and the associated seals?
Is that a job that can be tackled from underneath the car or does the front end have to come off?

[Audisthesia]

Common issue....

That's what THIS is for...

[Akakios]

As in, that dumb plastic coolant pipe and the associated seals?
Is that a job that can be tackled from underneath the car or does the front end have to come off?

Service mode is required to access anything really on the front end except the drive belt for the alternator.