Positive Crankcase Ventilation: Description and Operation

Crankcase Ventilation System






Engine: M70 (V12)

The M70 engine is equipped with a new and different method of controlling the flow of crankcase vapors.

Similar to the BMW 6-cylinder ventilation system, the new system employs a liquid-vapor separator baffle in each cylinder head cover. Now however, a flow control valve (MBV) and a vacuum limiting valve (RSV) are also used for each cylinder bank.

Cylinder Bank I (1-6) crankcase gases flow into the inlet air plenum of Cylinder Bank II (7-12) and vice versa. This point is extremely important to remember when diagnosing oil consumption or air/fuel ratio (DME fault code # 10) complaints.

As of January 1989 production, the MBV and RSV valves have revised flow characteristics. See S.I. 11 01 90 (3015) for complete details.






Each MBV flow control valve is regulated by intake manifold vacuum against internal valve spring pressure. As stage I and stage II move within the MBV housing, the cross sectional flow area changes, regulating the volume of gases to the inlet plenum and vacuum to the crankcase.

This action assures the correct amount of crankcase vapor flow while limiting the vacuum exerted on the M70 crankcase.






Typical MBV valve operation is shown corresponding to the three common engine operating conditions.






The RSV vacuum limiter valve is a simple one-way flap-type valve. Ordinarily, the flap or diaphragm is held in the closed position. When high manifold vacuum is present, the diaphragm is lifted connecting the engine vacuum with the incoming air charge.






During periods of engine operation which produce high manifold vacuum (closed throttle deceleration etc.), the RSV valves provide a manifold vacuum limit. Excessive vacuum is vented to the air passage between the hot wire air mass meter and the DK throttle valve. This action provides a peak vacuum "ceiling" thus allowing the MBV valve to accurately modulate crankcase gas flow based on engine load.