Audi Cars 1999-05

Crankcase Ventilation System


Description & Operation

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Fig. Diaphragm-type crankcase ventilation valve

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Fig. On the 1.8L engine, the PCV valve is located on the intake bellows

To send oil fumes and crankcase blow-by gasses back into the engine for burning, all engines are equipped with some type of crankcase breather control system.

The crankcase ventilation system consists of a ventilation hose connecting a fitting on the top of the cylinder head cover to the air intake tract, and a secondary hose attaching the intake manifold to the ventilation hose. A baffle under the hose fitting on the valve cover prevents excessive oil vapors from escaping and a potential flashback from occurring. The small hose connecting to the intake manifold has a built-in restrictor orifice, which essentially creates a controlled vacuum leak to the intake manifold. When the throttle opening is small and manifold vacuum is high, crankcase oil vapors are drawn directly into the intake manifold. When the throttle opening is large and manifold vacuum low, some of the oil vapors flow through the large hose to the airbox. Later engines use this same system, except the ventilation hose attaches to a plastic baffle on the cylinder block, instead of the cylinder head cover.

The 2.0L engines used in the Passat models are equipped with a replaceable pressure regulator valve located in the top of the valve cover which functions much the same as previously described system. This valve controls the ventilation of the crankcase and cylinder head into the intake tract. The diaphragm control valve maintains a constant balance against manifold vacuum, keeping blow-by vapor flow at a constant percentage of the total intake air volume.

VR6 engines use a simple crankcase ventilation system, similar to the 4-cylinder engines. A single hose connects the cylinder head cover to the intake tract, ahead of the throttle body. A heater element is contained within the hose, which prevents the vapors from icing over in cold weather. As with all engines, the cylinder head cover contains a metal mesh flame trap that contains any ignited vapors during the event of an engine backfire.

On all systems, there is no maintenance required other than to check for vacuum leaks and clogged hoses. When removing hoses for inspection, take note of the direction of the restriction valves. Replace any clogged or cracked hoses.