Positive or negative crankcase pressure?

[Betelgeux]

Hi,

Re my 85 318i-- manual says bad oil filler cap or dipstick seal can create a
vacuum leak. OTOH, I thought the crankcase was under positive pressure(hence
Positive Crancase Ventilation valves) due to blowby gasses.

Any wisdom here?

[floyd rogers]

Betelgeux <1@2.3> wrote

Think of it this way: The PCV valve sucks air from the crankcase
(which is directly connected to the oil filler in the camshaft cover
via the oil drain-back paths from the head). Now, if you leave the
dipstick loose, or the oil filler cap off, the PCV valve will suck
extra (clean/outside) air into the intake manifold, DOWNSTREAM
of the airflowmeter and throttle plate, ruining the fuel/air mixture.

FloydR

[Betelgeux]

Thanks. But I think crankcase gasses drawn in through a PCV would upset the
fuel/air mixture as much as fresh air would. But could be wrong about that,
as the O2 content is different.

And I'm even wondering whether it is possible to generate vacuum in the
crankcase using only the downward motion of the pistons, since that same
motion would tend to compress the existing air in the crankcase and raise
its pressure an equal amount. And blowby gasses would only add to that.

Can anyone add anything? Don't make me go out there and actually put the
palm of my hand over the oil fill opening :-)

[x]

Betelgeux <1@2.3> wrote in message
<81acnl$3p8c$1...@newssvr03-int.news.prodigy.com>...
>Hi,

>
>Re my 85 318i-- manual says bad oil filler cap or dipstick seal can create
a
>vacuum leak. OTOH, I thought the crankcase was under positive
pressure(hence
>Positive Crancase Ventilation valves) due to blowby gasses.
>
>Any wisdom here?
>
>

Why in the hell do you people even care?

pull the dipstick out and put your toung over the hole, you dipstick.

Here's a piece of advice: Don't screw with things that you don't kow about
and don't need screwing with.

[Technical]

BMW Engines are considered to be "self-cleaning" by Appling vacuum to the
engine through the intake system. The "Positive" crank ventilation is drawn
up into the intake through the valve cover hose to the intake. The engine is
considered a vacuum chamber and should be in negative vacuum during normal
running conditions or at idle.

If you have an air leak in the engine system via the oil cap, a bad valve
cover gasket, the air leak created is missing from the engine air intake
that passes through the air flow meter is diminished by the leak and the
ECM/AF meter "thinks" your engine is running at a lower idle and adjusts the
fuel injectors and timing to compensate. This will cause poor running,
hesitations and premature failure of the emissions system, like the
catalytic or O2 sensor.

I came up with this idea and BMW recognizes it for testing the engine
ventilation in their test procedures.

A good way of finding air leaks in BMW engine is to cut your old (or new)
BMW fuel filter in half. You'll notice you have a funny looking cup with a
hose fitting on the end.

Remove your air flow meter intake hose and stick the large cut-off end into
the air boot were the air flow meter plugs in to the intake boot. On the
other end is the fuel line fitting. Apply air pressure, (30 to 50 PSI is
good) to this end for a few seconds. It should hold pressure for 10 seconds
or more. Too much pressure can blow out bad or defective seals, so be
cautious on the amount of pressure you use.

With a 3/4" heater hose or equivalent, listen around the engine, top to
bottom, front to back for leaks by sticking the open end of the hose in your
ear.

Most air leaks are found in the engine seals and gaskets. Some smaller leaks
that cause intermittent rough idles are found in the vacuum hose diagram
connections.

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Scott

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