Audi A8 Mystery Coolant Loss, Hidden Leak Warning

Having worked on a 2005 Audi A6 with a similar issue, I can share some relevant experience. With basic mechanical knowledge, I initially tried finding the coolant leak myself but eventually needed professional help. The issue turned out to be a faulty connection at the coolant reservoir sensor, causing false readings and the check engine light. The repair cost was 95 Euro including diagnostics and parts. Since fixing the wiring connection, the warnings stopped and coolant levels remained stable. However, your case might be different as actual coolant loss is occurring. If you are regularly topping up the coolant, there is definitely a leak somewhere in the system. Common areas to check include: Water pump seals, Head gasket integrity, Radiator fluid connections, Coolant reservoir cap and housing, Hose connections and clamps To better assist, could you provide: Engine size and type, Approximate coolant loss rate, Any unusual smell from exhaust, Whether coolant loss occurs when engine is hot or cold, If you notice any white smoke from exhaust This information would help narrow down potential causes and suggest the most likely repair path.

My A8 has 49977 KM and was last serviced 3 months ago. The issue started showing shortly after. When checking around the water pump area, I noticed slight dampness but no obvious dripping. The coolant loss is about 500ml every 2 weeks. No white smoke from exhaust, and the radiator fluid level drops mainly after longer drives. After reading about similar cases, I pressure tested the cooling system and noticed the radiator cap wasnt holding proper pressure. The coolant reservoir also shows hairline cracks near the top. Could be worth starting with these simpler fixes before diving into more complex repairs.

Thanks for those details about your A8. This actually mirrors what happened with my A6 after I fixed that initial sensor issue, I also discovered secondary problems that were causing actual coolant loss. The symptoms you describe strongly point to the failing radiator cap and cracked coolant reservoir as the primary culprits. The pressure test results confirm this. When the radiator cap fails to maintain proper pressure, the entire cooling system operates inefficiently, often causing coolant loss through weak points like those hairline cracks you spotted. In my case, replacing both the cap and reservoir solved about 90% of the coolant loss. The remaining seepage around the water pump area eventually needed attention too, but it was minimal compared to the main leak sources. Key points from my repair experience: New radiator cap: 35 Euro, Complete coolant reservoir replacement: 180 Euro, Replace all connected radiator hoses while access is easy, Full system pressure test after repairs: 60 Euro Make sure to use the correct pressure rating for the cap (should be marked on the old one) and get OEM or equivalent quality parts. The coolant loss should stop completely within a week of these repairs if there arent any other underlying issues. These fixes are much more cost-effective than diving straight into water pump or head gasket repairs, especially given the symptoms you described.

I appreciate all the detailed feedback and suggestions. After reviewing everything, I think Im going to start with replacing both the radiator cap and coolant reservoir. The pressure test results and symptoms match up with what others experienced, and it makes sense to try these simpler fixes first. Taking your advice about doing it all at once, I will have the radiator hoses replaced during the same service. The total cost should be around 275 Euro for parts plus labor, which seems reasonable compared to major engine work. My car has been in great shape otherwise, so hopefully addressing these cooling system components will resolve the constant coolant loss. I plan to monitor it closely for a few weeks after the repair and will do another pressure test to confirm everything is sealed properly. The dampness near the water pump might need attention later, but it makes sense to fix the obvious issues first and see if that resolves most of the coolant loss. Thanks for helping me develop a logical repair strategy that starts with the most likely causes.