Coolant expansion tank

The expansion tank accommodates changes in coolant volume and, depending on the circuit layout, helps remove air and gases from the engine cooling system. The air chamber provides room for expansion; the pressure cap limits system pressure and allows pressure equalisation as the coolant cools. A leaking tank or faulty cap can cause coolant loss, trapped air and localised overheating. Simply topping up does not address the underlying cause.

Always select expansion tanks using the vehicle's chassis number. Visual comparison or selection based solely on the model name is never sufficient. Across MAN TGL, TGS and TGX and Mercedes Atego, Actros and Arocs, tank capacity, mounting points, connection diameters and level sensors vary by version. Engine families such as MAN D08 and D26 or Mercedes OM 936 and OM 471 help narrow down the cooling configuration, but do not determine fitment on their own. The exact circuit configuration must also be checked for buses such as Lion’s City and Citaro and vans such as TGE and Sprinter.

Distinguish between pressurised expansion tanks and unpressurised overflow reservoirs: they are not interchangeable. Check the number of bleed and return connections, the filler neck, cap locking mechanism and sensor interface. Heat-stabilised plastic housings must withstand thermal cycling and the specified coolant. Material type, temperature resistance, nominal capacity and permissible working pressure must match the vehicle configuration. The cap's opening pressure and associated tolerances are specific to the version; never fit a cap with a higher pressure rating to prevent coolant discharge.

Typical faults include hairline cracks along weld seams, brittle hose spigots, distorted filler necks and leaks around sensor seals. Dried coolant traces often reveal small leaks that only occur when hot. A stuck float or corroded connector can trigger a coolant level warning even when the tank is correctly filled. Repeated discharge through the cap can also indicate overfilling, inadequate bleeding or gas entering the cooling system from the engine.

1. Check when cold: only open the system once it has cooled down and is no longer pressurised. Compare the level with the cold fill mark and inspect the housing, mounting brackets, hose connections and cap seal for cracks, chafing and deposits.
2. Perform a pressure test: use a cooling system pressure tester with a suitable adapter. Pressurise the circuit to the specified test pressure without exceeding it, and record pressure changes and test duration in accordance with the workshop instructions. Locate any traces of moisture; pressure loss alone does not prove that the expansion tank is faulty.
3. Test the cap and level monitoring system: measure the pressure cap's opening pressure separately and check its vacuum relief function. Compare the sensor status shown on the diagnostic equipment with the actual level. Only measure the power supply, wiring or switching resistance in accordance with the sensor wiring diagram.
4. Investigate recurring excess pressure: check the bleed lines for blockages. If combustion gas ingress is suspected, use a suitable gas detection test and carry out further engine diagnostics; air bubbles alone do not prove a head gasket leak.

Replace cracked or distorted plastic tanks rather than repairing them with adhesive. Check rubber mounts and brackets: mounting under stress causes new cracks. Fit suitable seals and the specified hose clips, observe tightening torques and avoid lateral loads on connection spigots. Specifically check whether the pressure cap and level sensor are included or need to be replaced separately.

Fill with coolant that meets the manufacturer's approval; colour is not a reliable specification. Check the mixing ratio using a refractometer suitable for the coolant type. Bleed the system in accordance with the vehicle procedure, using vacuum filling equipment if permitted. After a complete warm-up and cool-down cycle, check again for leaks, correct level indication and the specified cold fill level.