Mercedes-Benz W204 coolant leak
Mercedes-Benz W204 CDI: where does the coolant usually leak from?
On the OM651 engine the most common leak points are the water pump and the oil filter housing base joint. The pump can leak so little that no leak is visible even though the coolant level drops. Both parts are relatively quick to replace.
Mercedes-Benz W204: what other leak should I suspect when I cannot find the leak point?
A hose connection can also leak, where the seal sits in a groove inside the joint, so replacing the hose alone is not always enough. Coolant can also run onto the intake manifold from a cracked connector piece or a gasket. The leak point is sealed or the part is replaced.
Mercedes-Benz W204: what if the coolant disappears and there is air in the system?
It can point to the cylinder head gasket or the cylinder head. In a fully warmed-up engine the coolant must not rise above the cap and there must be no bubbles. If it rises or bubbles, the cylinder head may have to come off, so have a workshop investigate.