Sci-fi_Wasabi

Well, there are not too many mentions of head gasket leaks in this car, and the fluid is not a slam dunk yellow, but a little less blue. Just want to make sure your test is executed 100%, before you go down the route of throwing sealers in there etc. I don't know the symptoms of your car or anything about it, but there's a lot of sources of coolant leaks in all this aged heat soaked plastic we have in our cars.

S4'ed

I had the same issue on a 2005 VW/Audi 1.8t. Car would run fine, parked it overnight. Next morning, at cold start-up, one cylinder would misfire for about 30 seconds, then ran fine the rest of the day.

The head gasket failed, allowing a little bit of coolant to leak into that cylinder, as the hot system stays pressurized overnight, until the engine cooled down. No mix of oil into the coolant, or coolant into the oil. Slow drop of coolant level. Car had never overheated.

I confirmed this was the problem as I removed all the sparkplugs on a cool engine, and boroscoped all the cylinders, the cylinder with the cold-start miss had a wet piston top (from the coolant).

Traded that car the next week.

MP4.2+6.0

Continuing prior reply--not from 4.2 but other prior Audis--iif you have a coolant into combustion chamber head gasket leak, in pulling spark plugs that have been in the vehicle a decent number of miles, the one in the leaking cylinder will basically look steam cleaned...becuase it effectively is. The other ones will look normal with the slight deposits, tannish dry if motor in good condition, oily if older and tired, or a separate issue in a given cylinder. Never had to use a chemistry set to figure this out, though last I actually saw a blown head gasket on an Audi was 40 years ago. Having pulled a head on a 4.2 (port) for piston oil burning reasons, 1) it was do-able (hardest is unbolting exhaust connections) and 2) it didn't seem likely as a fail point for water given construction, decent water jacket spacing, quality head gasket construction/materials, etc.

Rob09A8

+1 "there's a lot of sources of coolant leaks in all this aged heat soaked plastic we have in our cars."

I noticed OP had recently replaced his aux coolant circulation pump himself so without context about what's going on with the car it's assuming a lot from one type of test.
Age of the fluid test medium also seems to be a variable.
Other tests/procedures can be done such as compression, leak-down, looking at the condition of the glow plugs [especially if one is steam-cleaned compared to the others] to get a more overall health check of the engine-- and back to S-FW's comment.

Waterflow

I've replaced the aux pump recently (1-1,5 months back) but overall has been performed flawless. No white smoke, no check engine light on due to misfire or any other reasons and never have noticed any coolant spots under the car. However, I would say in the last maybe 2-3 months I've noticed the coolant level started very slow to drop.tdropping. That is when I decided to run this test to determine if a potential head gasket leak or EGR cooler could be the issue.
I will appreciate any opinions on this matter before I will schedule a visit to Audi since the closest dealer is 7 hours away.

Rob09A8

That's a very long way!
As this is the D3 side of the forum [generally 4.2 petrol non turbo engines] MP4.2+6.0 would be better to direct you into looking for sources of coolant leaks on a D4. Google searching aimed at the D4 forum TDI engine might be more fruitful.

EDIT: Actually I assumed you had a diesel from the earlier comment of the failed chemical test being green rather than yellow but it wasn't stated what your engine was or I missed it.
My understanding is that yellow is the failure color on a petrol engine test.

2nd EDIT It is a gas/petrol engine and Waterflow is going to repeat the test with some more room in the expansion tank when he has the chance.

MP4.2+6.0

That reference is clear as mud. Do you mean the fine print after his chemistry picture?

Unless the Board has chased away the non North America crowd, still also gotta field TDI's in Europe and elsewhere, both V6 and V8. In D3 era, there are V8's TDI too, which had no North America counterpart. Yes, D4's in Europe and elsewhere too, and briefly 3.0TDI in USA before Dieselgate.

I would urge folks again to read the one and only sticky on this board. Way too many parts of this thread recently where vehicle, motor, year, location--all the most basic stuff--are missing, not clearly stated. Doesn't help it's a 10 year old thread either that seems to have morphed again.

Rob09A8

This is a D4 4.0 GasTurbo...posting in the D3 section
You've got a S8 D4 haven't you?

MP4.2+6.0

Read my reply again. Your own reply was ambiguous, and could be taken as insulting.

Rob09A8

Well apologies if that came off rude. I didn't mean it to be. I agree with you entirely about all those factors you mention in your edited response. The D4 in the D3 side was the confusion.