Ha Ha, looks like I could found this post a little bit too late and I may have made a mountain out of a molehill. Great info here from everyone who's contributed and I thought I'd share my experience so far with my 2013 MK7 Golf 2.0TDI currently sitting at 115,000km on the odometer (seems to be the trend here). The tail below is a bit long but I find detailed posts like these very insightful (although maybe not always helpful) when trying to diagnose issues I'm having. The general consensus here is that the heater matrix is the prime suspect but still, maybe my experience is of use to others.
About 3 months ago I turned my ignition on and got the low coolant alarm. I had no idea at this stage where it was leaking from so I topped up the coolant with some demineralised water. Like everyone else here, this reoccurred but after the second time I tracked it down to the expansion reservoir relief cap, suspecting a head gasket leak might be the issue, I got a gas sniff tester. I initially got a positive green result from the cold expansion tank before even starting the engine but dismissed that initially as I could not get a positive test again while running the engine as hard and as hot as possible (while stationary). I repeated the test a few more times over the coming days and each time only got the test to go green when testing cold right after opening the bottle, not while running. Thinking this might just be a TDI thing I tried it on my other Golf (also a TDI but an older EA188) and the result was exactly the same, green on cold, all good during an extended engine run at 2500RPM with AC running on a 42degC day.
So now I'm thinking maybe it really is a TDI thing so I repeat the test on two other TDI's belonging to friends (one a Skoda EA288 and the other an Audi EA188), no colour change whatsoever !!
So I did 1.5bar pressure test on the coolant system and it passed the 30min VAG recommended test duration and continued to hold pressure for 3 hours more before I stopped. I didn't have an adaptor to test the cap itself, so I decided to just replace the pressure relief cap on the expansion vessel as it seemed like the “low hanging fruit” option. I thought after 12 days of constant checking that I was home free but at the end of a 300km round trip I opened the bonnet and there was fresh coolant still running down the bottle. So, I get a large rag and some gloves and open the brand-new relief cap, no hiss or pressure was released but when I tested the air inside, bang, it instantly turned yellow, not green, yellow!
All doubts now gone and being the impulsive home mechanic I am, I proceed almost immediately to rip the entire engine apart to see if it’s the head gasket. I did do a bit of research before I came to this conclusion and although I did consider the EGR Cooler and Charge Air Cooler as a possibility for gas entering the system, I didn't think these were as likely as the head gasket given my yellow gas test result.
So long story short, the head gasket didn't have any signs of a cylinder to gallery breach, both engine block and cylinder head were dead straight (less than 50um over the surface which was the smallest feeler gauge I have). While I had it all off, I also cleaned everything up and did a dye penetrant crack test on the cylinder head and the engine block and all four cylinders. Furthermore, Everything in the engine looks great, no build-up anywhere other than a small amount of carbon build-up at the top of the cylinders.
This is my first head gasket and while the EA288 CRBC is an amazing piece of engineering, it's ridiculously convoluted in terms of the cooling and electrical circuits encircling the engine. Teardown and rebuild so far has taken me almost 30 hours and I estimate 10 more to complete the job (I'm doing the timing belt and water pump this weekend and that should be it before refilling the coolant and changing the oil). I've spent $460 aussie dollars on OEM parts (renews including the head gasket, head bolts, and seals for various bits and pieces). The timing belt and water pump parts cost about $600 but that was already a planned job before the first coolant loss. Timing belt interval here in Australia is 120K, possibly even 105K if you believe VW Australia. I only use genuine VW parts.
I also spent about $200 on some special tools for looking the cams, again this was an investment I made for the timing belt job prior to the coolant loss occurring.
So that's my story so far, I guess the questions is, will it be just the head gasket or will it be the EGR Cooler (another $510), could it be the intercooler (another $1200) or just the heater matrix (a mere $285).
Anyone who's exhausted all other avenues and it looking to take on the head gasket themselves, I'm happy to share or answer any questions you have. I'll keep you posted on my car once it running again. Dame, I'll be pissed if it turns out to be just the heater matrix, you don't even need a damn heater in this country