Diesel engines intended for dusty areas were equipped with a pressure control valve between the breather pipe and the valve covers at the factory. Volkswagen Buses that were exported to countries where this was not a problem, are without this pressure control valve, the hose is directly connected to the valve covers.
With the rubber mentioned here, the connection between the valve covers and the pressure control valve is made. The rubber cannot withstand temperature changes; over time it dries out, resulting in cracks. If the grommet is torn, outside air will be drawn in, which must be avoided.
Check the condition of the rubber at each maintenance. If hairline cracks appear, it is time to replace the rubber.
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