2007 S8 Coolant reservoir/tank or valve leak???
 

Pleading for the experts to comment:

All,

I am having an issue with the coolant reservoir/tank on my D3 S8. On more than one occasion the tank is leaking or a valve on the underside is leaking badly and coolant and water are draining out the bottom of the car. When it happens it comes on strong and dumps a bunch. The temperature gauge never budges off dead center where it is at normal operating temperature. When I open the hood and remove the cover the water/coolant inside the reservoir is bubbling and boiling and steam is escaping from the underside. From the top you cannot see anything.

Once it cools down, I can add distilled water back up to the full line and it can go for a few days with no issues and then it does it again. On more than one occasion it happens after I arrive back home and park in the garage. I come back out in a couple hours and its lost a bunch of what I replaced all over the floor. Like I stated earlier though, the car never overheats or anything like that. I do get the warning on the dash to add water but that's the extent of it. What the heck is going on here?

Yes, the reservoir's failure mode is exactly what you describe. The crack (likely at a seam) opens when hot. They are not expensive (Audi-wise that is).

Mr Bally,
Thanks for the quick reply. OK, if the tank has a crack in it.........how in the world do ya take the damn thing outta the car? I tried to remove it the other day to examine it but I could not get it to come outta its tight little space after removing the only nuts I could see. What am I missing?

I recall a bolt from the bottom/side that required the fender plastic panel to be removed/peeled back to access.

Of course it does. Then I will need to remove the backseat and the drop the exhaust to access another small plastic nut. Nuthin is ever easy. Thx Mr. Bally

Let me back up on the diagnostic here...
 

I would not assume it is the cooling reservoir until you find or see an actual leak in it. The symptoms you describe could be the thermostat too, and I think there have been some of those failures on S8's posted to the board. If so, the coolant blowing out the bottle area (overflow) can be a symptom, but not necessarily the problem. Your temp gauge readings do suggest it might be a crack at the bottle though. You could have simply a bad cap to the bottle as well.
On getting the reservoir bottle out, a guess is it is like the W12. Is it located behind the first firewall integrated with the plenum area and under a cover near the master cylinder? If so, those do come straight out but it's a super tight fit. If I recall from mine, it could be the trick is you actually have to remove the plastic trim/box around it at the same time.

OK, I am at a loss......for now. I finally figured out how to remove the tank. Relatively easy once you know to remove the fender liner, pull it back and expose the hose and the sensor. Here's the problem. I removed it and inspected the plastic top to bottom and there is NO CRACK. so, I reassembled the whole thing and started the engine for about 10 mins until it reached operating temp. I let it idle for another 10 mins while viewing the bottom side of the reservoir and it did NOT bubble or leak. I then replaced the fender liner and wheel and of course it started to bubble and steam again but I couldn't tell where it coming from because I already had the wheel back on. However, after knowing the plastic seems to be solid and crack-free, I am thinking thermostat. The only problem with that is the temp gauge never leaves dead center (normal operating temp). Could it be the thermostat? If so, whats involved in replacing the thermostat?

Another (ugly scenario) diagnostic:
 

Let me ask another not-having-a-good-day scenario/diagnostic: is the coolant in the bottle it's normal pinkish red, or has it got some definite oil slime to it or floating near the top? And, have you opened the oil cap to see if it is just normal dark oil look, or if it has the milky emulsion sort of stuff going on. Either of course are among the pointers to a blown head gasket, depending on exactly where the cylinder seal breach is. Those can pressurize the bottle, and sometimes rupture it too. Not saying or predicting; hopefully just excluding that scenario.

For what it's worth, most of the time the seam between the two halves starts to leak under pressure (when hot). There is a tool that replaces the cap which allows you to pressurize the cooling system while cold to find leaks. You can make one out of a spare coolant tank cap and pipe fittings to pressurize with shop air as well. That said, it is cheap enough to just replace the reservoir.

However, like MP4.2+6.0 says, it could be the thermostat, thermostat housing, heck, a leaky water pump could be your issue as well. You really have to determine where the leak is coming from to be sure. If you are not certain that you can do it, it may pay to take it to a qualified shop.

You seem mechanically inclined. Maybe leave the fender cover and lower engine cover off. The car is driveable this way. Especially in the Metroplex.

Need a service manual and/or someone who has been thru it.
 

I don't recall an S8 thermostat in recent board memory, though I'm thinking there may have been one or two further back. Search for it perhaps.

From the parts listing, it actually looks like it is shared with the later 4.2 FSI motors. Thus, see if there is any info for the 4.2 FSI if you want, as well as diagnostically. Apparently in a large assembly with the water pump on one side (front) and then thermostat on the side. Thermostat apparently part of a plastic casting and all one piece. Don't rely on these, but it may be part 4 here: Audi - A8Q - 2007 - Engine, Clutch Teile Katalog (Pats Catalog) In turn that looks like: Search thermostat - 079121115BA - Thermostat - ES#2785772