A5 Mystery Coolant Loss: Hidden Leak Warning Signs

Having dealt with a similar issue on my 2013 Audi A4 2.0T, I can share some relevant experience. As someone with moderate experience working on these engines, coolant loss without visible leaks often points to a few specific areas worth checking. First, there might be air pockets in the cooling system. When I had this issue, my mechanic found that the issue wasnt actually a leak, but rather a faulty connection at one of the coolant temperature sensors. The cable connection had deteriorated, causing false readings and system pressure issues. The repair for my sensor connection issue came to 95 Euro, which included diagnostic time and parts. However, your issue could be different, as these engines can develop problems with: Head gasket (internal leak), Radiator cap (pressure loss), Radiator hoses (microscopic cracks), Water pump seals, Expansion tank hairline cracks Before suggesting more specific solutions, could you share: Any warning messages besides the check engine light? Does the coolant loss rate increase with highway driving? Have you noticed any white exhaust smoke? What color is your coolant when you check it? These details would help narrow down the potential causes and provide more targeted advice based on your specific situation.

Thanks for the helpful replies. After 184470 KM on the clock, I had the cooling system pressure tested at my last service. The mechanic found hairline cracks in the water pump housing causing slow coolant seepage. The radiator fluid was also showing signs of contamination. The head gasket seems fine with no signs of oil mixing or white exhaust. The radiator hoses and cooling fan are working properly. Looking at around 750 Euro for a complete water pump replacement including labor. Going to schedule the repair next week before it gets worse.

Thanks for the update on your A5. That matches what I experienced, my water pump also developed hairline cracks around 180000 KM. The slow seepage can be tricky to spot since the radiator fluid often evaporates before forming visible puddles. The 750 Euro quote sounds reasonable for a complete job. When I had mine done, they also recommended replacing the radiator cap and flushing the system to remove any contaminated coolant. The flush helped eliminate air pockets that could have caused problems later. Make sure they check the radiator hoses during the repair since they are already accessing that area. In my case, the upper hose showed some minor wear and it made sense to replace it preventively while everything was apart. The good news is that once the water pump is replaced, the system should be trouble-free for another 100000+ KM based on my experience. Just keep an eye on coolant levels during the first few weeks after repair to ensure everything sealed properly.

Thanks for the advice. Just got back from having the water pump replaced, ended up costing 785 Euro total since they found the coolant reservoir also had a hairline crack that needed fixing. The mechanic showed me the old pump and you could clearly see where it was seeping around the housing. They did a complete system flush and replaced all the radiator hoses while they were in there. The cooling fan and radiator cap were still in good shape so those stayed. The contaminated coolant definitely needed to go though, it was pretty gunky when they drained it. Its been about a week now and the coolant level has stayed steady. No more check engine light and the temperature gauge is rock solid. Definitely feels good having this fixed before it turned into a bigger problem.