MP4.2+6.0

Not clear to me how much fluid he is losing, nor do i think he seems to know /measured. Or, at least not spelled out here except qualitatively. Belly pan holds a lot more than a few ounces often times, and if it dumps on road and/or burns off on hot engine you never see it hit the ground. Been there (not) seen that on recent V8 and W12 Audis. One lowly way to end run it sometimes is park car on a real slope. It can force the accumulated coolant to run off, especially after a long drive where the system is pressurized and then pees it out.

Belly pan examination remains one of the best diagnostic tools on Audis, as lowly as it is. Up there with VCDS overall, particularly as the vehicle ages. Yes, a mechanic can do the same, though probably blow it off unless savvy to the technique or lots of Audi experience. My own general Audi repairs on the bigger motors and cooling systems would put odds on: a bad hose connection (or on later Audis like these a weak O ring at a plastic fitting connection that uses a metal clip retainer to fasten it instead of older style worm drive clamps); cracked fitting thermostat cover (if plastic like 4.0T); maybe an overflow/reservoir bottle issue; maybe a failing thermostat which could also be causing some overheating and blow off of coolant out the reservoir drain (where the cap lets it out). Some of those may show from above, but it takes patience with a strong flashlight and knowing the pinkish tinge and consistency to look for. Cool engine usually better to be able to feel around more for wetness too. Never say never, but the 4.2's and 4.0T's seem to have few head gasket issues even at quite extended miles.

For OP, scan for cooling codes with VCDS too, or mechanic can. You mention a tool, but I suspect it may be sort of generic OBD-II. Not convinced you will find any though; my recent experiences are when there are codes you sort of already know--a light comes on or message flashes and sometimes a CEL, and/or cooling fans go into jet engine mode as a fail safe to protect engine.

yuvalw123

Had same problem with my 2011 4.2 A8L
In the end it was the crankcase vent valve , all the way at the back of engine.
the whole fix was 250$ ( parts and labor )
was very hard to find the small leak.( and only when car was running )
good luck

chris.barnett.2011.a8

i have had suspicions about something on the backside being the issue. But can’t really get back there to check without pulling some stuff off.

chris.barnett.2011.a8

as far as rate goes, idling I don’t really lose any. Can sit for over an hour idling without warnings or dropping. Start driving and that’s a different story. Not consistent loss but driving and the car moving around seems to cause it to be worse.

I do get a code for lean left bank from time to time. What I had read was bad o2 sensor on that side and the mA readings look slightly off on my tester.

A8L_New_England

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Again, Chris, how much are you losing? Can you smell it?
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MP4, as he said, "it was getting expensive chasing this with actual coolant", I would expect that to be more than the belly pan would hold without dripping all over the place at some point. I certainly agree that he (his mechanic) should have a look under it!
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MP4.2+6.0

You are missing the point I have made a couple times now: it often evaporates, burns off as it spills down hot parts, or blows off with the flow through air at road speeds. It doesn't just sit there as a puddle as it were unless it's a huge leak like burst hose. I had two slow coolant leaks for years on the D3 W12. First dealer never even found, but I could tell from CPO purchase from them they had been poking around--coolant bottle was marked for fluid height, and various work order notes. I found it was a cracked radiator core right near end of warranty, and only with belly pan off and spotting slightly wet area far forward. Second was a leak near front of motor post-warranty. Telltale pink residue on bottom of oil pan from leading edge of engine, and on pan. Never 100% sure I got that one. Neither one hit the ground, ever, yet especially on the second scenario the marks were on the pan as well as motor.

Current one is D4. Have found coolant on ground twice after 200 mile drives, and realized it was only w/ car parked on slope immediately after those destination drives. No leaks ever visible on ground otherwise. Just did oil change last week and gave it another close look. Found slight marks on pan--all dry--directly below what I matched up to be thermostat housing. Then looked very carefully at housing and saw minor evidence of some kind of leak--also dry and just residue marks. Concluded it may be some very slight crack or seal issue with the plastic thermostat housing, which is a previously flagged issue for 4.0T's. Without the belly pan careful exam, I would never have spotted a source.

chris.barnett.2011.a8

I put a half to a gallon back in after driving for a couple of hours. Just using water for now.

Again the hard thing is that it doesn't seem to leak when it is just idling still. I would think something serious like a gasket or hose would be leaking while parked to so its the only while driving that I have a hard time tracking down.

A8L_New_England

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My point is that there would be some sign underneath (even without removing the pan) if he is losing that much fluid. ...and smell...
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A8L_New_England

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Chris, have the pan pulled and do a pressure test.
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chris.barnett.2011.a8

Will do. I don’t know if this helps but when I ran it parked it today I heard steam. Here is a video looking in towards the beck left of the motor. Will add link, video is to big to attach