Engine bleeding

The engine breather system controlledly removes blow-by gases, oil vapours and water vapour from the oil sump. On MAN and Mercedes trucks, buses and vans, this system is essential for stable crankcase pressure, protection of seals and correct operation of the air intake and emission control systems. The oil sump is the lower part of the engine that contains the engine oil. While the engine is running, a small amount of combustion gases always leaks past the piston rings into the oil sump. These are known as blow-by gases. These gases increase the pressure in the oil sump and contain oil vapours and unburnt fuel. The engine ventilation system ensures that the resulting excess pressure is discharged, the gases flow back to the engine and oil is separated from the gases by means of an oil separator. Depending on the engine family, emission standard and vehicle application, connection diameters, hose routing, oil separators, pressure regulation characteristics and mounting points may differ. Therefore, always identify parts using the chassis number.

Older engines, such as certain Mercedes OM 364, OM 366 and OM 402 engines, may feature an open crankcase ventilation system in which crankcase gases are routed directly outside or to a separate outlet. More modern diesel engines, including Mercedes OM 906, OM 457, OM 460, OM 471 and MAN D08, D20, D26 and D38, usually use a closed crankcase ventilation system. After oil separation, the gases are returned to the air intake, generally upstream of the turbocharger.

A closed crankcase ventilation system may be designed as a PCV system with a pressure control valve, as a central vacuum or low-pressure system, or with a separate filter and oil separator. Many heavy-duty engines also use turbulence separators or labyrinth ventilation systems: internal channels slow down the gas flow so that oil droplets are separated from the vapour flow through centrifugal action, impact and condensation. The separated engine oil flows back to the oil sump, while the cleaned crankcase gases are routed to the intake. An incorrect oil separator or breather hose can result in insufficient oil drainage, incorrect vacuum in the crankcase or contamination of the turbocharger, intercooler and intake manifold.

A clogged filter element, blocked labyrinth chamber, hardened hose, cracked diaphragm or defective pressure control valve restricts flow. Excessive blow-by caused by worn piston rings, damaged cylinder liners or increased cylinder pressure can also exceed the capacity of the ventilation system. Therefore, always check whether the problem lies in the ventilation module or is the result of internal engine wear.

Excessive pressure and too many blow-by gases in the oil sump cause several actual types of damage. Initially, oil leaks occur at valve cover gaskets, crankshaft seals, sump seals and turbocharger return lines. Reduced lubrication may then occur because engine oil is carried into the intake or because oil mist is insufficiently separated. This increases oil consumption and reduces the available lubricating film on main bearings, connecting rod bearings and piston pins. In addition, reduced engine performance may occur due to oil contamination in the charge air cooler, contaminated pressure sensors and incorrect air mass values.

Condensation water, acids and unburnt fuel components in inadequately discharged crankcase gases accelerate engine damage through corrosion, including in the valve cover, crankcase area and steel breather lines. In closed systems, oil vapours in the intake can also cause emission-related problems: increased soot formation, EGR contamination, abnormal diesel particulate filter regenerations and fault codes relating to air mass or turbocharger pressure control.

First check the oil level, the correct engine oil specification and any visible leaks. Then inspect hoses, quick couplings and connection flanges for kinks, cracks and oil traces. Measure the crankcase pressure using a suitable manometer or low-pressure transducer at the specified measuring connection, at idle speed and under load. Compare the measured vacuum or overpressure with the vehicle-specific factory value. Clearly positive crankcase pressure indicates restricted ventilation or excessive blow-by.

If in doubt, remove the filter element or oil separator and check for sludge, ice formation, carbon deposits and diaphragm damage. Replace complete ventilation modules when the filter and control valve are integrated; separate hoses or sealing rings should also preferably be renewed when softened by engine oil. Braem can assist with chassis-number-based parts identification and technical identification for MAN and Mercedes applications.