S6 Coolant Level Mysteriously Dropping Despite No Leaks

Had the same issue on my 2019 S4. After multiple coolant top-ups, the problem turned out to be a failing radiator cap that wasnt maintaining proper pressure, combined with tiny air pockets in the cooling system. The symptoms match perfectly, no visible coolant leak under the car, no white smoke, but constantly dropping coolant levels. The radiator fluid would drop every few days despite regular filling. The fix required: Pressure testing the entire cooling system, Replacing the faulty radiator cap, Bleeding the cooling system to remove air pockets, Inspecting all radiator hoses for potential micro-cracks This was not a serious repair compared to head gasket issues. The pressure test confirmed the head gasket was intact. Would recommend getting it checked soon though, as running low coolant repeatedly can lead to bigger problems. My S4 has been running perfectly since the repair with stable coolant levels. Best to have a professional diagnose it properly with a pressure test to pinpoint the exact issue.

Thanks for sharing your experience. I had almost identical symptoms with my S6 a while back, coolant mysteriously disappearing without any obvious leaks. Was really worried about the head gasket at first but luckily it turned out to be a much simpler fix. The cooling fan had developed a small crack and the water pump seal was starting to fail, causing the system to slowly lose pressure. Once the radiator hoses and cooling system were properly pressure tested, they found the real culprit. Do you remember roughly what you paid for the full diagnostic and repair? Just curious if costs have changed much since I had mine fixed. Has the repair held up well with no engine overheating or other cooling issues since?

Thanks for asking about the repair costs. For my S4, the total diagnostic and fix came to 95 Euro, which covered tracing and repairing a faulty cable connection that was causing the cooling system pressure issues. The repair has held up perfectly, no more engine overheating warnings or coolant reservoir issues since the fix. The radiator fluid level has remained stable after they properly bled the system to remove all air pockets. I check the coolant level monthly now just to be safe, but haven't needed to add any in the 6 months since the repair. The cooling system maintains proper pressure and the engine temperature stays consistently normal, even during longer drives. It sounds like your issue was a bit more complex with the cracked fan and water pump seal. These cooling system problems can be tricky to diagnose, but thankfully both our cases were caught before any serious engine damage occurred.

I took it to my trusted mechanic last week and the total came to 480 Euro for parts and labor. The water pump seal replacement was the bulk of the cost, plus they had to order a new cooling fan assembly. The coolant reservoir needed a thorough flush too since some debris had collected from the failing pump. The repair took about 4 hours total, including pressure testing all the radiator hoses and bleeding the cooling system properly. The mechanic showed me the old parts, the crack in the fan housing was tiny but definitely causing the pressure loss. Been driving it daily for the past week and the coolant level has stayed perfectly stable. Engine temperature gauge stays right where it should be, and the check engine light is gone. Seems like catching it early prevented any major issues, definitely worth the cost to avoid potential engine damage. Still keeping an eye on the coolant level out of habit, but the repair has completely resolved the problem. No more mysterious coolant disappearing acts or warning lights.