[Craig380]

My lad's BMW 420i had a sudden major coolant loss when he was coming home from work last night. The vent tube which connects between the cylinder head and the coolant expansion tank broke off at the cylinder head.
Luckily he was only a couple of miles away and so was able to nurse it home without it overheating.

Turns out this coolant tube is failing in significant numbers, it's made of a hard plastic which becomes brittle from repeated engine heat-cycles and cracks (causing minor coolant weeps / loss) and eventually breaks completely (causing major unexpected coolant loss. It's common enough that it's a free 'service call' in the US market, with a BMW US technical service bulletin describing it and stating that customers should be contacted to get it replaced.

However, despite the BMW US technical service bulletin, BMW UK claim not to have heard of the issue, and deny seeing any such failures on cars over here. What's more, they will not even look at the car without charging a 'diagnostic fee' before undertaking a repair, as they refuse to accept external diagnoses. Ridiculous when the diagnosis is a broken f*cking coolant pipe. Striping b*st*rds.

The car is 4.5 years old and 47K miles. If anyone has a 2, 3, 4 or 5 series with the B48 2.0 petrol engine made between 2015 and 2019, I'd strongly recommend getting this pipe checked.