Crankcase ventilation

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

1. exclude the possibility, that too much oil pumping in this mode (when the inlet manifold has high affluence);

2. reduce the impact of crankcase pressure on idle run (long-term fuel trims, 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 AND at Homogeneous injection mode – if the engine working mode changes (idle run RPM is going down for a moment), you feel the significant air sucking in while the oil addition cap is opened – the membrane is ruptured.

If we monitor the live data for long-term fuel trim and integrators, in case of intact membrane there will be no significant changes in long-term trims and integrators in the first moment (at least 5 .. 10 seconds), even if 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 still will allow to “suck out” the air to inlet manifold, but it will not happen immediately, and there will be no momentous engine working mode changes, when opening the oil addition cap.


Several nuances are acceptable:

1. there can be acoustic noise. This noise is created due to a high difference of pressures, 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. first time opening the oil addition cap, there can be feeling, that the air is sucked in or excessive pressure can be observed. When the cap is closed, pressures in crankcase and inlet manifold are even (the membrane doesn’t cover the valve perfectly, so minimal air flow via it is possible). If the cap is opened for a moment (few seconds), closed and after a moment opened repeatedly – the feeling of sucking can’t be present anymore. If it’s still here – the membrane is ruptured.

A typical feature in case of ruptured membrane: immediately after opening the cap, long-term trim integrators are changing rapidly, reaching +30 % in few seconds, offset long-term trims are immediately overwritten.


What are the consequences of the ruptured membrane?

1. 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. in an idle run, the engine has significant differences of long-term trims from 0, they are “flying”. Also, fuel mixture problems for a cold engine (vibration, misfires, etc.) are possible, problems with maintaining idle run mode and also problems with the quality of fuel mixture after changing the driving conditions.


(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 should be replaced
  5. Open the oil filling cap and observe integrators – if they are changing rapidly and reach 20 .. 30% in few seconds: the valve is not closed. Clean crankcase ventilation valve or replace membrane (valve cover for N43/N53 series engines)


[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: