RS3 Mystery Coolant Loss Baffles Multiple Mechanics

A failing head gasket is certainly possible, but first check the radiator cap and radiator hoses for proper sealing. Had similar symptoms on my 2018 Audi S3 where the engine overheating warning kept appearing. Initial inspections showed no visible leaks, but pressure testing revealed the radiator cap wasnt maintaining proper pressure, allowing coolant to escape through vapor. The cooling fan was also cycling more frequently than normal, which is often a sign of system pressure issues. The problem became obvious when white smoke appeared from the exhaust during cold starts. After thorough testing, it turned out to be a combination of a faulty radiator cap and a hairline crack in one of the radiator hoses near the clamp. This was a serious issue that required immediate attention to prevent engine damage. If left unchecked, these symptoms can indeed progress to head gasket failure. Recommend getting a proper pressure test of the cooling system. While doing that, have them inspect all hose connections and the radiator cap seal. These components often fail before more serious issues develop.

I experienced the exact same issue with my RS3 last summer. Had the same symptoms, coolant mysteriously disappearing and air pockets forming in the system. The mechanic found that coolant was leaking past a worn radiator cap seal and discovered a tiny crack in one of the radiator hoses. The cooling fan was also running more than usual which made me really worried at first. But fixing these relatively minor issues solved the problem completely. No further coolant loss since the repair. Did you end up getting it fixed? What was the final cost for your repair and have you noticed any other issues since then?

Finally got it fixed last week and it turned out to be much simpler than expected. The main issue was a faulty sensor connection that was causing incorrect readings, leading to air pockets in the system. The mechanic found that a cable connection to the coolant level sensor was loose and corroded. The total repair cost was 95 Euro which covered the electrical fix and a thorough inspection of the cooling system. Since the repair, the coolant level has remained stable and the warning lights have stopped. They also pressure tested the entire system to rule out any radiator hoses issues or potential head gasket problems. The cooling fan now operates normally and the engine temperature stays consistent. The radiator fluid level hasnt dropped at all in the week since the repair, which confirms it was just the sensor connection causing false readings rather than an actual leak.

Thanks for sharing your experience. After getting both opinions, I took my RS3 to a certified mechanic last Thursday. The diagnosis showed a hairline crack in one of the radiator hoses and a failing radiator cap, pretty much exactly what was suggested here. The repair was straightforward, they replaced both radiator hoses as a preventive measure, installed a new radiator cap, and performed a complete system flush to remove any air pockets. The total came to 320 Euro including parts and labor. The car has been running perfectly since then. No more coolant loss, and the check engine light has stayed off. The cooling fan now cycles normally, and the radiator fluid level remains consistent. Such a relief it wasnt a head gasket issue as initially feared. Always good to start with the simple fixes first.