PCV system question

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JM - 12 Mar 2007 22:16 GMT

Hi all,

I've got a 1999 Nissan Altima, and today while I had the breather tube
from the valve cover to the intake hose off with the engine running, I
noticed that while there is suction at the valve cover connection at idle,
there's also a regular "exhaling" there as well, ie a puff of air maybe 4 -
6 times a second. Although the net effect is a vacuum, at idle or above,
should there be a steady vacuum there? If so, what would the positive
pressure indicate? Excessive blowby on one cylinder? A leak somewhere? Or
normality?

Thanks in advance!

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HLS@nospam.nix - 12 Mar 2007 22:55 GMT

> Hi all,
>
[quoted text clipped - 6 lines]
> pressure indicate? Excessive blowby on one cylinder? A leak somewhere? Or
> normality?

The PCV valve functions such that less crankcase gas is pulled into the
induction
system at idle than at increased RPM. This helps insure that idle operation
is not
strongly affected by the gases being introduced. At full speed, the engine
can
run more smoothely even though more crankcase gases are being pulled in.

There will be some pulsation in just about every case I have ever seen.
Whether
it is normal or not is yet to be decided.

You may be having some leakage into the crankcase. Some is normal. It is
hard
to guess from the information you have given.

Are you burning excess oil, or have you noted a decrease in power or
economy?

You can run some pressure tests on the individual cylinders to see the
relative condition of the rings and cylinder walls, valve seal, etc.

A compression test is cheap and easy to do. A cylinder bleed down
test may take a little more effort.

Most people, AFAIK, just go with the flow until and unless the engine
operation gets to be difficult to tolerate. There isn't much you can do
about
it anyway, except to go inside and refurbish the weak component.

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JM - 12 Mar 2007 23:34 GMT

> There will be some pulsation in just about every case I have ever seen.
> Whether
[quoted text clipped - 4 lines]
> Are you burning excess oil, or have you noted a decrease in power or
> economy?

No, not really. The car has always had a faster than normal idle, and a
hesistation off idle, that I've been trying to get to the bottom of. The
PCV valve is one of the last things I've not been able to eliminate as a
cause so that's why I was wondering. I actually had the breather hose off
to plug it briefly to see if it had any impact on either symptom. It did
affect the idle, but I imagine that's to be expected, however the hesitation
was no better or worse.

Anyway, I was just mostly curious as to whether that was an indication of
anything in particular being wrong, but it sounds like it's not necessarily
abnormal. Thanks for the reply!

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AS - 13 Mar 2007 23:52 GMT

Everytime a piston goes down in the cylinder, there is some pressure
increase in the crankcase, somewhat compensated by the movement of the
other pistons. What you see is normal.

> Hi all,
>
[quoted text clipped - 8 lines]
>
> Thanks in advance!

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JM - 13 Mar 2007 23:59 GMT

Thanks, I wondered about that, but I would have thought the other pistons
moving would cancel out the effect.

> Everytime a piston goes down in the cylinder, there is some pressure
> increase in the crankcase, somewhat compensated by the movement of the
> other pistons. What you see is normal.

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Nate Nagel - 14 Mar 2007 02:59 GMT

It does, overall, but there's inertia of the air in there to consider,
so at any one point at any one instant you could have either pressure or
vacuum. In other words, say you have a two cylinder engine (to be very
simplistic.) When piston one goes down, piston two goes up. However,
the air has to physically move between those two spaces, and there will
be a slightly higher pressure underneath piston one than piston two in
that instance.

nate

> Thanks, I wondered about that, but I would have thought the other pistons
> moving would cancel out the effect.
>
>>Everytime a piston goes down in the cylinder, there is some pressure
>>increase in the crankcase, somewhat compensated by the movement of the
>>other pistons. What you see is normal.

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