The E46 cooling system uses many plastic parts, and they age from heat and pressure cycling. Two parts fail often.

The expansion tank cracks and breaks: often with no warning, while driving, and the coolant is gone within minutes.

The impeller of the water pump on the six cylinder engines is plastic, and it often breaks between 100,000 and 160,000 km. The circulation then stops, and the broken pieces scatter through the system: the repair does not end with replacing the pump.

This is not a factory defect but the nature of the design: on the E46 the cooling system is a wear item. If the service book shows no replacement, count on changing the plastic tank and the water pump. Repairing the cylinder head of an overheated six cylinder costs a lot, and it can cost more than the car.