I have an issue with my A6 that, through WEEKS of searching, I have not been able to solve. Every forum I could find and even the stealership itself can't figure it out. A little bit of background first. The car had been losing coolant from the day I bought it. Expansion tank crack was the source. Replaced it to no avail. 10 minutes later, I'm back in my driveway watching it spay from the bottom this time, right around the sensor. Faulty tank right? Nope. Replaced it again. This time I built a seal to pressure test the tank. All good. Installed it only to start a leak from the hose directly under it. At this point, I reviewed all of the likely suspects. For one, the coolant was bright green. As most know, this wasn't correct. So, wrong coolant. Next was the review of maintenance record. Other than a bill of sale, nothing. Timing belt and water pump may have been done, but judging by the coolant, probably not. I had at this point decided to attempt the timing belt, water pump, all pulleys and tensioners, just to be safe. I purchased the Ultimate Blauparts kit and a few extras, and the crank locking pin and cam lock bar. Installation was a lot less complicated than I anticipated. Fair instructions and forum guys helped a lot! So, most front 'O' the engine parts relplaced. Including thermostat, coolant hoses and heater control valve. It was leaking. Basically, everything in the coolant circuit was replaced, except for the radiator, heater matrix and temp sensor (tested good.) I flushed the heater matrix, it ran clean, and temp sensor is spot on. So, my current issue is that my new coolant expansion tank cap is the only place that coolant is leaving the system. NEVER overheats when idling. Only when driven over 40 it seems. Or over 4k if you like. Here is the rub. Head gasket right? Nope. Leakdown test and compression tests confirm less than 10% loss on all cylinders. All within 5% of each other. Almost unheard of in my limited experience with cars. I'm stumped. No exhaust gasses present in the coolant. No mixing of oil or coolant either way. No strange exhaust or smells or leaks! Runs like a champ other than coolant loss. Here is my theory, the heat isn't as hot as I would expect. Suggesting a restricted heater core. Flows free though. As retarded as it may sound...an air bubble in the heater core? But I can idle for hours, put it under load, but no coolant reduction in the expansion tank. ONLY when I drive it around. My next experiment is to get some extra hose and fittings and bypass the heater core. Then see if I can duplicate the issue. PLEASE save me from my retardedness! I'm not a certified mechanic by any means, but I'm not half bad. This is driving me nuts! As well as broke. Anyone with any info on this situation, I would greatly appreciate your help. Sorry for being so long winded!