Cooling water and filter

Coolants dissipate combustion heat and protect coolant passages, cylinder heads, water pumps and radiators against freezing, corrosion and cavitation. Coolant filters remove circulating contaminants where fitted to the engine variant. Some versions also contain conditioning additives. Choosing the wrong coolant or filter can reduce heat transfer and damage seals, even when the measured freeze protection still appears adequate.

Always select coolants and filters using the vehicle’s chassis number, together with the engine variant and maintenance requirements. Visual comparison or selection based solely on the model name is never sufficient. For example, requirements for MAN D20 and D26 engines or Mercedes OM 471 engines may vary according to the year of manufacture and technical configuration. Vehicle-specific approval also applies to MAN TGE, Mercedes Sprinter and buses such as the Lion’s City and Citaro; a coolant filter is not fitted as standard.

Distinguish between IAT (Inorganic Additive Technology), OAT (Organic Acid Technology) and HOAT (Hybrid Organic Acid Technology). These technologies use different corrosion inhibitors; colour does not establish whether they can be mixed. Check the required MAN 324 specification, including its subtype, or the applicable Mercedes-Benz operating fluids specification sheet. The approval determines compatibility with materials including aluminium, cast iron and elastomeric seals. Use concentrate with water of the specified quality or a ready-mixed coolant. Approximately 50% ethylene glycol by volume typically provides freeze protection down to around −37 °C, but product data and manufacturer requirements remain authoritative. Adding more glycol does not improve cooling: viscosity increases and heat capacity decreases.

For coolant filters, the thread, seal diameter, installation space, filtration rating, permissible differential pressure and temperature resistance are determining factors. Distinguish between mechanical filters without additives and conditioning filters with a specified additive charge. An incorrect charge can cause overdosing; never fit a conditioning filter solely because the connection fits.

Brown discolouration, sediment and deposits may indicate corrosion, unsuitable mixing water or mixed coolants. Depleted corrosion inhibitors increase the risk of pitting corrosion and cavitation damage, including to wet cylinder liners where fitted. A clogged filter can restrict flow through its branch of the circuit. An oily film calls for investigation into possible engine oil cooler or cylinder head gasket leaks, for example; simply changing the coolant will not resolve the underlying cause.

1. Allow the engine to cool and never open a hot, pressurised circuit. Check the level, maintenance history and any leaks, and inspect a coolant sample for particles or phase separation.
2. Measure the glycol concentration with a calibrated refractometer using the correct glycol scale. Use suitable test equipment to check the pH and, where specified, inhibitor concentrations against product-specific limits. Freeze protection alone does not confirm that corrosion protection remains effective.
3. Carry out a pressure test when cold using a hand pump and pressure gauge at the specified test pressure. Do not exceed the permissible system pressure; also test the pressure cap separately. Investigate any pressure loss for external and internal leaks.
4. Where test points are provided, check the differential pressure across the filter under the specified operating conditions. Compare it with the filter limit. In the event of overheating, also check thermostat operation, water pump flow rate and radiator fouling before replacing components.

First repair the leak or eliminate the source of contamination. Drain the coolant in a controlled manner and dispose of it as waste. If the existing coolant is unknown or incompatible, flush the system according to the manufacturer’s procedure; residual flushing water affects the final mixing ratio. Replace the specified filter and seals, clean the sealing surfaces and follow the installation instructions. Fill and bleed the system according to the vehicle-specific procedure, especially on buses with long heating circuits. After warming up and cooling down completely, recheck for leaks and verify the level and concentration. Record the product approval, mixing ratio and replacement date to prevent topping up with the wrong coolant.