The brief
The 218i had been leaving a small puddle on the carpark floor every morning, enough that the owner had been topping up the coolant about once a week, and the temperature gauge had started creeping up when the car was crawling in traffic. He brought it in before it overheated for real.
The radiator is where the coolant dumps its heat: hot coolant runs through a fine aluminium core in the airflow and comes out cooler. That core takes years of pressure cycles and road grit, and eventually it can develop a pinhole, small enough that the leak is slow, but enough to drop the level and to mean there is not quite enough coolant in the system to keep up when airflow is low, like in stop-start traffic.
The diagnosis
A pressure test on the cooling system traced the leak to a pinhole in the radiator core itself. The hoses, the water pump and the expansion tank all checked out clean, so it was the radiator and nothing else.
There is no patching a leaking radiator core in any way that lasts, and a core that has started to weep will only get worse. The right fix was a new radiator, which also gives the cooling system a fresh, full-capacity core rather than one that has been quietly losing coolant for who knows how long.
The work
The cooling system was drained, the radiator hoses disconnected, and the failed radiator lifted out. A new BMW-spec radiator went in, all the hoses reseated with fresh clamps.
Then the system was refilled with the correct coolant and bled of trapped air, and pressure was held on it to confirm the new radiator and every connection were sealed before the car was road-tested.
The outcome
No drips, the coolant level holding steady, and the temperature gauge stable through traffic and out on the open road.
The 218i went home with the cooling system back to spec and the morning puddle gone. For the owner that is the end of the weekly top-up and the end of watching the gauge in slow traffic. And replacing the radiator before it failed outright meant no overheating event, which is the difference between a planned cooling-system job and a much bigger engine repair.