BMW crankcase ventilation valve and separator

The crankcase ventilation system is used in all nowadays engine. Its task is to seize the vapors of oil and fuel and drain them to inlet manifold for repeated burning.

Unfortunately, the understanding, how exactly this knot works – most often it’s limited with “leveling the pressures”, which is not true.


Crankcase ventilation consists of 2 basic parts:

1. oil separator;

2. valve (membrane).


The task of the first knot: separate oil/fuel fumes from liquid fraction. For this purpose, centrifugal or labyrinth type separators are used. In both cases-the liquid fraction (because it has a higher density than gases) is separated, and only the gas fraction is drained to the membrane. This not is quite primitive, and only the separator of labyrinth type requires technical maintenance. If the “walls” of the labyrinth are covered with residues – oil slag, dirt – it losses it’s performance (the walls become rounded, not straight).

The membrane works in the following way: it works as a valve, and in case of great depression in inlet manifold turns off the crankcase recirculation. This function has several reasons:

1. exclude the possibility of much oil pumping into the intake manifold in this mode (when the inlet manifold has high depression);

2. reduce the impact of crankcase gases (affects offset type long-term fuel trims/LTFT) and pressure (affects throttle adaptations) to idle run – creation of fuel mixture.


For older engines (till N52), the crankcase ventilation is placed outside the engine and look’s like pictured above. For N53, the crankcase ventilation valve is built in on the top of the cylinder block, opposite cylinder No.5.

The most typical defect of crankcase ventilation system – rupture of rubber membrane.


How to check working abilities of crankcase ventilation?

At idle (Valvetronic – disabled; important for N series engines, except N43/53) AND at Homogeneous injection mode (important for N43/53 series engines): if the engine working mode changes (idle run RPM is going down for a moment; the engine is shaking) and/or you feel the significant air sucking in while the oil addition cap is opened – the valve is not closed (but should be).

If we monitor the live data for offset type long-term fuel trim and integrators, in case of correctly working CCVV there will be no significant changes in long-term trims and integrators in the first moment (at least 5 .. 10 seconds) when we open the oil addition cap. Of course, the membrane is not intended for such working mode, and there is a possibility, that the large pressure difference between its cameras (crankcase and intake manifold) still will allow to “suck out” the air to the inlet manifold, but it will not happen immediately. There will be no immediate engine working mode changes when opening the oil addition cap.

If the membrane is ruptured or airtight of a valve is lost (for example, air is sucked in by the perimeter of the membrane), integrators will change their values if/when the ventilation hole (placed on the top of the CCVV) is closed manually*. An engine will have serious problems with idle at start-up (uneven run, shaking, misfiring), errors relating fuel mixture (lean or rich), usually offset type LTFT are not confirmed (their values are 0.00 mg/stk); they are significantly increased (above 1.00 mg/stk) or their values are floating – are significantly different (relative difference can reach +/-0.5 mg/stk or even more) for cold/hot engine, right after the start of the engine and after few minutes, etc.

*remove plastic cover of the engine and close the ventilation hole using your finger


Several nuances are acceptable:

1. there can be acoustic noise (when the oil adding cap is opened). This noise is created due to a high difference of pressures (between crankcase and intake manifold), while the system tries to suck in the air via membrane to the inlet manifold. Still, the membrane has to be checked – the leakage is possible also via membrane rupture, not only valve;

2. opening the oil addition cap, excessive pressure can be observed. Higly increased pressure in crankcase is an indication of wasting of piston rings, pistons and engine block itself.


A typical symptom in case of:

  • not closed valve: immediately after opening the cap, integrators are changing rapidly, reaching +30 % in few seconds; in case of huge air amount, sucked into the intake manifold – in addition: offset type long-term trims are immediately (within 5 .. 10 seconds) overwritten (to the ”positive” side);
  • ruptured membrane: when CCVV ventilation hole is closed*, integrators are changing rapidly and reaching -30% in few seconds; in case of huge air amount, sucked into the intake manifold – in addition: offset type long-term trims are immediately (within 5 .. 10 seconds) overwritten (to the ”negative” side).

*remove plastic cover of the engine and close the ventilation hole using your finger


What are the consequences of problems with CCVV?

1. if  the valve is not closed: the oil consumption grows. Accordingly – the load for the exhaust system and Lambda probes is growing. If the vehicle has CO and NOx catalytic converters – they are damaged fast;

2. if the membrane is ruptured: fuel mixture problems for a cold engine or right after the start (vibration, jerking, misfires, etc.) are possible; problems with maintaining idle run mode and also problems with the quality of fuel mixture (mixture is rich or lean, integrators are significantly different from 0; their values are outside +/- 10 % corridor) after changing the driving conditions. Usually offset type LTFT are not confirmed (their values are 0.00 mg/stk) or they are significantly different from 0 (more than +1.00 mg/stk) or unstable (regularly changed/overwritted).

Note: if CCVV lost its airtight (due to the ruptured membrane or air sucking by perimeter) usually valve stays allways open and the engine has increased oil consumption (in addition to unstable idle, misfiring problems).


(1) Body

(2) Rubber membrane

(3) Spring

(4) To intake manifold

(5) From oil separator (crankcase)

(6) Ventilation hole


A: Small depression in the intake manifold (Mid/high torque)

B: Large depression in the intake manifold (Idle at Stratified charge; Idle/low torque at Homogeneous injection; overrun mode: M series engines; N43/53. N52/42/46/54/55 with deactivated Valvetronic)


The practice.

  1. Turn on the engine at idle;
  2. Check for Homogeneous mode (for N43/N53 series engine) [1]; deactivate Valvetronic (for N52/42/46/54/55);
  3. Open fuel trim menu using INPA [2];
  4. Close ventilation hole[s] at the top of the membrane (using finger) [3], if integrators are changing rapidly, air sucking thru hole[s] is observed – membrane is ruptured and CCVV should be replaced;
  5. Open the oil filling cap and observe integrators – if they are changing rapidly and reach +30 % in few seconds: the valve is not closed. Clean crankcase ventilation valve or replace CCVV.


[1] open ../F9/F1 (using INPA loader 2.023)

If engine performs at a Stratified charge (Lambda 2.0 .. 2.5), press F1 and switch engine to Homogeneous injection mode

Note: after the test don’t forget exit forced Homogeneous mode: enter menu repeatedly and press F5


[2] fuel trim menu ../F5/F6

marked: integrators


[3] ventilation hole: