: Oil getting sucked in through PCV Valve: Old Mechanic's Tale?!

07-28-04, 05:33 PM
Someone on this group mentioned that they like to keep their N* engine oil level below full to prevent the oil from being sucked into the intake manifold via the PVC valve.

That got me thinking - how it this possible ?

The oil is in the oil pan (bottom of the engine). PCV Valve is on the very top of the eninge (in the highest position of the cam cover). It is probably over a foot higher than the oil level (even if the engine was way overfilled) and in a totally different cavity.

Even if the engine is running, the oil pump sends some oil via the passages into the engine head where it exits from the cam bearings and from lifters. It is not a a large quantity.

No matter how much oil is in the pan, oil pump will not send any more oil to the top of the engine. It only sends as much as it can pass through the passages (and whatever doesn't get dumped at the pressure regulator).

So, normal or even excess quantity of the oil in the pan shouldn't affect the quantity of oil being delivered to the top of the engine. Therfore, there will not be any more oil available on the top of the engine for the PCV valve to "suck".

Any comments ?

Peteski :hmm:

07-28-04, 09:19 PM
I believe the theory is that over filling it brings it to a level where the counter weights on the crank whip it up and create a mist that gets pulled into the PCV. I know my oil usage decreased when I stopped keeping it at the full mark.

07-28-04, 11:27 PM
Ranger is right.
Peteski, some of your premises are wrong . The oil pan is the same cavity as the head, there is no seperation except the distance. The N* does limit the oil flow to the head so this problem is minimal in a N*. The crank whipping the oil keeps more in the head area and more pulled into the PCV.

07-29-04, 04:55 PM
I guess I am confused...

AFAIK, on my 93 N*, the PCV valve is on the right head (rear). The only connection to the crankcase would be through where the timing chains are. That is quite a labyrinth through which the oil mist woudl have to travel...

So, what you guys are saying is that the oil mist is just like fog in the air and it travels through the timing chain passages to the PCV ?!

Peteski :bonkers:

07-29-04, 08:03 PM
That's right.

The engine should be designed to have a minimum amount of fog but if the crank contacts the oil you get foaming and fog. The PCV is putting a vacuum on the crankcase so if there is a lot of oil floating around it will pull it out. The timing chains would throw some of that up top too.

Worn rings do about the same thing. Put a lot of pressure into the crankcase and blow the oil around.