RE: Engine blowby

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi Vangelis,

Pressurised crankcase is a normal condition.  If you consider the pistons when they move
up and down in the cylinders are just as effective on the underside as on top.  In an
emissions compliant engine the crankcase is a sealed compartment so the pumping action of
the pistons tend to pressurise the crankcase and the vented oil vapour is usually taken to
a separator and then to the inlet manifold.  The vacuum in the inlet manifold relieves the
pressure in the crankcase.  When you remove the oil filler cap you loose the vacuum and
the crankcase vents to the atmosphere.  The true test is to insert a sensitive pressure
gauge into the crankcase with the oil filler cap fitted and see what pressure exists under
these conditions. If the rings have excessive leakage the net result is extra pressure
build up and oil venting.

In pre-emissions compliant vehicles the crankcase breather vented to atmosphere and some
oil loss due to vapours being blown off was common and this causes pollution.

I hope I have explained that sufficiently well.

John
Durban
South Africa
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