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[mark.adams]

I took the oil filler cap off with the engine running and there is a definite compression from within and fumes escaping.
Is this normal?

[Marks DTM Calib]

Yes, if the breatehrs are working ok (the small one in particular during idle) then there shoudln;t be any pressure in the crank case.

You have to consider that no piston-ring/piston/bore seal is perfect, all the rings have gaps for a start, so given the high cylinder pressures you do get some blow by gases from the combustion process.

These gases end up i the crank case and need to be dealt with and hence the breather setup.

[TheBoy]

As Mark says, normal to get a small bit of combustion fumes in the crankcase.  It shouldn't build up pressure, as the breathers should allow the pressure to escape.

If the breathers are not completely clear, this pressure has to go somewhere, and is normally the cam cover gaskets on V6 Omegas (or crank seals on ones fitted with Vectra cam covers)...

[mark.adams]

aaah i see... I'm guessing by removing the oil filler cap, the hole left is then bigger than the diameter of the breather pipes and thats why i noticed it. The breathers have all been cleaned out (several times).
Never removed it whilst engine running before so consequently never noticed. Thanks for putting my mind at rest guys.