Venting crankcase pressure?

[Unreal]

Never had an issue with a breather leaning it out. Crank case pressure is just blow by from the rings, not measured air by the maf. The maf knows what air is going into the intake manifold, not how much gets past the rings. Adding a breather on the crank case will not lean out the car. That would mean you are sucking air in through the valve cover, then backwards through the rings into the cylinders to lean it out, with a blower pushing air into the cylinders. You would have to have more negative crank case pressure than boost for any crank case air to get into the cylinders/intake and lean it out.

[ZO6 Bryan]

Originally Posted by Unreal

Never had an issue with a breather leaning it out. Crank case pressure is just blow by from the rings, not measured air by the maf. The maf knows what air is going into the intake manifold, not how much gets past the rings. Adding a breather on the crank case will not lean out the car. That would mean you are sucking air in through the valve cover, then backwards through the rings into the cylinders to lean it out, with a blower pushing air into the cylinders. You would have to have more negative crank case pressure than boost for any crank case air to get into the cylinders/intake and lean it out.

Not talking about the blowby gases so much. There is more of that at WOT. But at idle and part throttle the air entering the crankcase thru the PCV system has been measured by the MAF. You need to research how a PCV system works. Look at the car. Follow the airflow. Then you will see what I mean.

[ZO6 Bryan]

Originally Posted by Unreal

That would mean you are sucking air in through the valve cover,

The PCV system DOES suck air in through the valve covers.

[tsnow]

Z06, are you running the stock setup or some type of catch can?

[PQ]

Originally Posted by crg

I had an oil cap vent on mine for about 3000 miles. Got on the gas one day and check engine light came on with an O2 sensor code. Brought it to the dealer and after checking it out, they said it was the breather throughout the voltages off and that the PCV had to be a closed system. Put the original cap back on and code has not returned. Has anyone with the filler cap breathers had this issue or is dealer full of it.

A tune would fix that issue.

Originally Posted by Unreal

Never had an issue with a breather leaning it out. Crank case pressure is just blow by from the rings, not measured air by the maf. The maf knows what air is going into the intake manifold, not how much gets past the rings. Adding a breather on the crank case will not lean out the car. That would mean you are sucking air in through the valve cover, then backwards through the rings into the cylinders to lean it out, with a blower pushing air into the cylinders. You would have to have more negative crank case pressure than boost for any crank case air to get into the cylinders/intake and lean it out.

If blowby at idle and slow throttle is an issue (which if it is you got serious ring issues) then you'd be correct. But in theory if you take your oil cap off and let air come into the crankcase, that is unmetered air and will go directly to the intake on the other side and AFTER the MAP sensor. So it should absolutely lean the mixture. In the stock setup the only air going into the crank case from that side is metered air after the MAP sensor on decel.

Originally Posted by ZO6 Bryan

The PCV system DOES suck air in through the valve covers.

The stock system does suck air but AFTER the MAP sensor. So metered air. If you put a breather on it to bypass the stock, clean side set up then it will be unmetered air and go directly into the other side and into the manifold AFTER the MAP sensor.

EDIT: I think that is what you were saying. My bad.

[Stephen12ZL1]

Bryan is correct about unmetered air entering and causing the 02 sensor to read a lean condition. If your tune is adjusted for the small difference, this isn't an issue.

[PQ]

The valve that was incorrectly put together that I bought for my car would let unmetered air in the filter cap but not allow blowby air to escape.

I cut it apart and bought a valve and reversed it until I get me a better one.

I have had idling issues for YEARS and was shocked to see the issue still existed even after a rebuild. Just found out yesterday.

[ZO6 Bryan]

Originally Posted by PQ

A tune would fix that issue.



If blowby at idle and slow throttle is an issue (which if it is you got serious ring issues) then you'd be correct. But in theory if you take your oil cap off and let air come into the crankcase, that is unmetered air and will go directly to the intake on the other side and AFTER the MAP sensor. So it should absolutely lean the mixture. In the stock setup the only air going into the crank case from that side is metered air after the MAP sensor on decel.

The stock system does suck air but AFTER the MAP sensor. So metered air. If you put a breather on it to bypass the stock, clean side set up then it will be unmetered air and go directly into the other side and into the manifold AFTER the MAP sensor.

EDIT: I think that is what you were saying. My bad.

Correct, thats how it works. Except you say MAP but mean MAF.

[Unreal]

Lean out if you still have the hose hooked up from crank case to intake, I was assume that is plugged when you are putting the filtered cap on. If anything if you plug the crank case to inlet and put a breather, it would run slightly rich because it wouldn't pull anything else.

My bad for not specifying that. I've always had that capped.

[jayno20]

OK, I am not understanding this completely...

I have an Apex catch can, that supposedly helps for venting some of this pressure, correct?

Now if I add an oil cap breather would that help even more? Or is that venting the same side that the catch can does?

[ZO6 Bryan]

Originally Posted by Unreal

Lean out if you still have the hose hooked up from crank case to intake, I was assume that is plugged when you are putting the filtered cap on. If anything if you plug the crank case to inlet and put a breather, it would run slightly rich because it wouldn't pull anything else.

My bad for not specifying that. I've always had that capped.

You have plugged the clean air side of the PCV system or plugged the lines that would connect to the catch can if you have one (the one that runs from the valley cover to the back of the throttle body)?

[ZO6 Bryan]

Originally Posted by jayno20

OK, I am not understanding this completely...

I have an Apex catch can, that supposedly helps for venting some of this pressure, correct?

Now if I add an oil cap breather would that help even more? Or is that venting the same side that the catch can does?

Catch can traps oil droplets, it doesn't vent.

[Unreal]

Originally Posted by ZO6 Bryan

You have plugged the clean air side of the PCV system or plugged the lines that would connect to the catch can if you have one (the one that runs from the valley cover to the back of the throttle body)?

My setup is a centri, 1/2" lines from each cover to vented catch can, valley to air filter, 3/8" line from valve cover to sump tank, vent on sump tank. So far from a factory setup.

You are correct though, you would want to run the "catchcan" line to premaf, or just run valley to a vented can and plug the inlet after the maf.

There are many ways to do it depending on how extreme you setup is. I was using dual 3/8" and still pushed oil out of the vented cap.

[ZO6 Bryan]

Originally Posted by Unreal

My setup is a centri, 1/2" lines from each cover to vented catch can, valley to air filter, 3/8" line from valve cover to sump tank, vent on sump tank. So far from a factory setup.

You are correct though, you would want to run the "catchcan" line to premaf, or just run valley to a vented can and plug the inlet after the maf.

There are many ways to do it depending on how extreme you setup is. I was using dual 3/8" and still pushed oil out of the vented cap.

Yep, some re-routing of the PCV system needs to be done when you're compressing air before the throttle body. Been there with my much older Vortech SC 95 TA.