Crankcase Ventilation: Overpressure or Underpressure

Crankcase Ventilation: Overpressure Or Underpressure

The combustion pressure presses gases out of the combustion chamber between the cylinder barrel and piston into the crankcase.

So that no overpressure occurs in the crankcase, the underpressure in the air intake system feeds the gases via the crankcase ventilation into the air intake system. The underpressure is limited by a pressure control valve.

Defects in the crankcase ventilation lead to high underpressure or overpressure in the crankcase.

The crankcase ventilation can be checked by measuring the pressure in the crankcase.

Causes of excessive overpressure
- Engine damage
- Obstruction of the crankcase ventilation
- Defective pressure control valve

Causes of excessive underpressure
- Defective pressure control valve

Symptoms and consequences of excessive overpressure
- Destruction of seals
- Slipping of dipstick (if present)
- Increased oil consumption
- Oil in the air intake system after the inlet point
- Oiled charge-air intake line on supercharged engines

Symptoms and consequences of excessive underpressure
- Destruction of seals
- Increased oil consumption
- Oil in the air intake system after the inlet point
- Rough idling speed
- Sucking or whistling noise from the engine
- Rising mixture adaptation values

Measuring accuracy of the pressure sensor
- Measuring range: 0 to 25 bar
- Absolute accuracy: 1 %, i.e. maximum deviation 250 mbar
- Resolution: 10 mbar

Target values for petrol engines
The pressure control valve sets the following pressure range:

Deviation to the current ambient pressure 0 to -30 mbar
On N73: 0 to -40 mbar

Component and measuring tolerances result in the following values for a proper state:

Deviation to the current ambient pressure: +20 mbar to -60 mbar
On N73: +20 mbar to -70 mbar

Values in the event of a defect for petrol engines
A defect causes significant deviations from the values for the proper state:
Crankcase ventilation "clogged": Ambient pressure +100 mbar
Crankcase ventilation "defective": ambient pressure -170 mbar

NOTE: Due to the relatively low accuracy of the pressure sensor (± 250 mbar), the ambient pressure must always be determined using the measurement system of the BMW diagnosis system.