Volkswagen Front Wheel Drive 1974-1989 Repair Guide

Crankcase Ventilation System


See Figure 1

To send oil fumes and crankcase blow-by gasses into the engine for burning, all engines are equipped with some type of crankcase breather control valve. Volkswagen uses three different types; a simple restrictor orifice, a spring loaded diaphragm or a Positive Crankcase Ventilation (PCV) valve.


On 16 valve engines, a breather is mounted to the side of the engine block near the oil cooler. The breather contains a baffle plate that allows the oil to condense out of the blow-by gasses and drain back to the engine. The large hose connects to the air cleaner so filtered air can be drawn in as required. The small hose connecting to the intake manifold has a built-in restrictor orifice, creating a controlled vacuum leak to the intake manifold. When the throttle opening is small and manifold vacuum is high, crankcase oil fumes are drawn directly into the intake manifold. When the throttle opening is large and manifold vacuum low, some of the oil fumes flow through the large hose to the air cleaner. The hoses should be checked for blockages and for cracks which could cause vacuum leaks.

On engines equipped with diaphragm control valves, the valve is mounted in the top of the cylinder head cover and connects to the intake manifold. The spring and diaphragm maintain a constant balance against manifold vacuum, keeping blow-by vapor flow at a constant percentage of the total intake air volume. Another hose connecting to the air cleaner allows filtered air into the engine as required. The hoses should be checked for blockages and for cracks which could cause vacuum leaks.

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Fig. Fig. 1: Crankcase vapor flow on engines equipped with a diaphragm control valve. Other systems are similar

Carbureted engines are usually equipped with either a restrictor or a PCV valve in the hose between the camshaft cover and the intake air elbow on the carburetor. The hoses and the restrictor should be checked for blockage and cracks. If equipped, the PCV valve can be checked as follows:

  1. Carefully pull the PCV valve from the hose attached to the camshaft cover.
  3. Start the engine, then place your finger over the end of the valve. You should feel a strong vacuum and the idle speed should drop slightly.
  5. If the valve did not operate properly, it should be replaced (not cleaned). The valve can be replaced by simply disconnecting the hoses attached to the old valve, then connect the hoses to the new valve.