PCV system question

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!

Reply to
JM
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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.

Reply to
<HLS

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!

Reply to
JM

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.

JM wrote:

Reply to
AS

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

Reply to
JM

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

JM wrote:

Reply to
Nate Nagel

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