Movano B loses coolant but no leak is visible

If the coolant level steadily falls, there is no puddle beneath the van and no emulsion is visible in the oil, check a component that costs little and is often missed during diagnosis: the expansion-tank pressure cap. It is not an ordinary cap. It contains a pressure valve that maintains positive pressure in the system and raises the coolant's boiling point. When the seal or valve spring ages, the cap releases pressure, allowing coolant to evaporate and disappear without any visible leak.

How can a faulty cap be identified?

Typical signs include an intermittent rise in temperature under load or when climbing, gurgling from the heater matrix after the engine is switched off, and a whitish deposit around the tank neck. A failed pressure valve may also leave the radiator hoses soft when the system is hot instead of noticeably firm under pressure. Never remove the cap when the engine is hot. A sound system is then under high pressure, and expelled coolant can cause severe burns. Test the cap with a pressure tester on a cold engine, not by unscrewing it by hand. The cap is a service item worth replacing preventively whenever the coolant is changed. If coolant loss continues after replacement, the next steps are a pressure test of the complete system and checks of the heater matrix, radiator and hoses. Only then should the cylinder-head gasket be suspected.