BMW 318/325/M3/525/535/M5 1989-1993 Repair Information

Crankcase Ventilation System


See Figures 1 and 2

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Fig. Fig. 1: View of the M42 engine showing the crankcase ventilation breather hose connecting the valve cover to the intake manifold

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Fig. Fig. 2: Crankcase ventilation system operation-M30 engine shown as example


All engines use a sealed crankcase ventilation system that does not allow fresh air into the crankcase. This means the crankcase is under vacuum at all times and any leak in the system is a vacuum leak. It is important to make sure all hoses are in good condition and the dipstick is firmly sealed in the tube. If the dipstick tube is not firmly seated, a vacuum leak will occur which may cause engine oil to be drawn into the intake manifold.

The crankcase ventilation system draws the crankcase vapors a connection on the valve cover. The vapors are drawn into the intake air stream and burned with the air and fuel mixture. This reduces the emission of hydrocarbons into the atmosphere.


See Figures 3 and 4

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Fig. Fig. 3: Crankcase ventilation connector-M50 engine

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Fig. Fig. 4: Unplug the hoses and check for blockages and cracked hoses-M50 engine

The crankcase ventilation system is essentially maintenance free. There are no mechanical portions that need to be adjusted, nor are there any parts that need to be replaced on a scheduled basis.

The only checks that can be made to the system is for plugged or cracked hoses and faulty connections. Pull the hoses off the connections and check for plugging. Inspect the hoses for cracking. Check that the hoses are fully seated on the connection points.