The brief
The Q7 had been needing a coolant top-up about once a week. A wet patch had been forming under the front of the engine bay. And a closer look showed coolant traces around the plastic expansion tank.
The owner came in expecting a radiator job, which is a reasonable guess; a coolant leak on a car this age is often the radiator. But weekly top-ups plus a wet patch right under the expansion tank, plus visible residue on the tank itself, points somewhere simpler and cheaper than the radiator.
The diagnosis
A pressurised cooling-system test traced the leak to a hairline crack along one side of the expansion tank itself. The plastic had gone brittle with age and heat, and a fatigue crack had opened up under pressure.
We checked the rest of the loop, because a coolant leak deserves a proper look before you commit to a part. The radiator, the hoses, the water pump and the thermostat housing all checked out clean. The expansion tank was the only failed component. Tank replacement was the entire fix, no radiator needed.
The work
Drained the cooling system, removed the cracked expansion tank, and fitted a new VAG-spec replacement with fresh seals and a new cap. The cap matters: it holds the system pressure that raises the coolant's boiling point, and an old cap on a new tank is a weak link.
Refilled with the correct coolant at the right ratio, ran the bleed cycle the proper way to clear the trapped air, and held pressure on the system to confirm a sealed result before the car went out.
The outcome
No drips. Coolant level holding cleanly. No smell.
The Q7 went home with the cooling system back to spec, and the owner paid for an expansion tank rather than a radiator, because that is what the diagnosis showed it needed. For the owner, that means no more weekly top-ups, no more wet patch on the driveway, and a car running at the right temperature, fixed for what the fault actually was.
Catching a cracked expansion tank at this stage also keeps the engine clear of the overheating risk that comes if a coolant leak is left to run.