BMW E46 Losing Coolant: Most Common Leak Points
E46 losing coolant, where is the leak usually found?
The most common leak points are a cracking expansion tank and its cap, the water pump and thermostat, and the upper and lower radiator hoses. A small hose that leaks only when cold can be found by checking the hoses. On the M43 the leak is often in the upper coolant pipe between the cover and the bulkhead.
Where does the coolant go when no leak is visible?
A small hole in the expansion tank can let coolant escape as vapour, so the leak is not visible from outside. An old cap may not release overpressure, which can break the tank. Behind the engine near the bulkhead there is a plastic connector that can leak; the plastic crumbles easily. The leak is found by pressure-testing the system.
What if the coolant level drops and white smoke comes from the exhaust?
Then coolant may be entering the combustion chamber, and the cause is often the head gasket. It can hold when cold but leak when warm. If no coolant is disappearing, white vapour seen in cold air is usually just condensation. The level is always checked on a cold engine.