breather cap: why?

[mugsy]

The PCV valve IS actually a check valve. If you don't believe me go buy a new one. Put each end to your mouth and then blow/suck. You'll find that the flow only goes one way.

Most PCV valves have spring that will close the valve under low vacuum situations. This stops the flow when the carb is close to WOT; the engine wants the air going thru the carb and not the engine when the "go pedal" is mashed.

The breather cap is the "inlet" to the system that let the engine block/crankcase breathe. The engine needs to breathe for various reasons: first, to consume the blow-by around the rings. Second, as the crank case heats up the air expands. Lastly, to remove the water vapor from the consensed water and vapors if/when the oil breaks down.

You'll notice the breather cap has a filter in it, or on other motors is taken from the filtered air stream. This is to reduce the dust going into the engine block.

 

[james singleton]

I was reading over this older post and wanted to ask a question concerning having a hose going from the breather cap to the top of the air cleaner. I have seen this set-up on various vechicles, and am wondering if there is a good reason for this set-up??? Is it strictly the fact that the air being drawn into the air breather cap is better filtered by having to go through the air cleaner filter first??? Jim

 

[JOHN G]

Just my 2 cents-I installed a cheap chrome valve cover on mine and eliminated the pcv. I have a cap at the front and breather at the back and all it does is dump oil vapor all over the place. I never had ths problem until the valve cover and current setup-I'll be installing the valve back in now along with a breather at the front of the valve cover like stock and expect the problem will most likely be gone.

 

[60s Refugee]

An air cleaner pulls less vacuum but still can suck out pcv gasses. Yes, you need a pcv of come kind.

Harry

 

[james singleton]

60'sRefugee, Maybe I am missing something here, but isn't the whole idea of a breather cap on the front of the valve cover for sucking in clean air and the PCV draws the gases out of the valve cover and into the intake manifold??? Wouldn't the rubber hose set-up I described coming from the top of an air cleaner over to the breather cap accomplish the same task, but the air would just be filtered better by going through the air filter first??? Anybody else care to comment. Thanks, Jim

 

[james singleton]

60'sRefugee, Soon as I wrote my post, and got to thinking, I know that is scary, I realized that indeed it would seem to me that the gases in the valve cover would not only be drawn out of the valve cover via the PCV into the intake manifold, but that gases would also be drawn out of the valve cover via the breather cap that has a hose going to the top of the air cleaner; since there is a vacuum created from the carb/air cleaner. Seems to me this would be the best set-up for getting rid of those gases in the valve cover. Anybody else care to comment?? Thanks again, Jim

 

[falcon fanatic]

James,
While there is vacuum in the air cleaner assembly, it is probably quite low relative to that of the intake manifold. The engine is not going to let itself implode, so if it needs to suck in air it likely has enough vacuum from the crankase to do so, even against the vacuum in the air cleaner assembly. And, you get filtered air going into your engine that way. That's how i have mine setup. PCV to the intake manifold, breather to the air cleaner.

 

[james singleton]

FalconFanatic, The reason I resurrected this subject, your subject, was that I recently installed a new air cleaner. My old air cleaner had the rubber hose going from the air breather cap to the top of the air cleaner as you describe; the new one does not. I have noticed in many pictures of our inline six engines that some people have this set-up, and other people do not. I am going to guess that like you said in your post, that maybe since the PVC vacuum removes the majority of gas fumes, that the breather cap to air cleaner probably has a more minimal effect of removing additional gases. I think I am going to modify, and re-install the hose from the breather to the air cleaner; can't hurt anything, right! Thanks, Jim

P.S. I might add that on my Chevy Inline Six (67'Chevelle w/ a 230ci engine), it has a PVC at the back of the valve cover, a cap at the front of the valve cover for adding oil, and a fitting w/rubber hose coming out of the side of the valve cover that goes to the top of the air cleaner.

 

[Jmonty]

That idea has been around for years, just a can with a hose running in and a hose running out. It's called a "catch can" and you can find them in any speed shop.

Actually, it's not exactly that. We use a similar filtration device on our air lines going into $75k+ machining centers. There is an oil/air separator that coalesces (sp?) the small oil & water droplets and allows them to drip into the catch part of the device. They also filter out particles larger than 5 or 10 microns.

I can see where that would be beneficial in an engine that has a lot of blowby. It would keep the incoming air/fuel charge from being contaminated with oil residue and hard to break hydrocarbons (the stuff that doesn't like to burn) and allow a better burn.

I don't think it's completely snake oil, but I'd have to try it out & see for myself. I might make one of those "condensator" things. I doubt the claims of high percentage fuel savings but it could keep the intake track cleaner.

 

[peeeot]

I have also seen a crankcase circulation and vacum system so tight, that when you took off the oil fill cap (or even pulled the dipstick tube) while the car was running it immediately died (vacum leak). See BMW and efficiency.

Every modern car that I have examined has had a pcv inlet in the fresh air intake stream, after the air filter. This accomplishes the same task that the filtered breather cap accomplishes, as well as the later system of having a tube from the breather run to the underside of the air cleaner, inside of the ring of the filter element. Each of these systems ensure that filtered air is drawn into the crankcase by the manifold vacuum which is regulated by the PCV valve. The filtration is important because the fresh air the vent system wants to inhale is likely to have dust in it that will contaminate the oil. Of course the cleaner the inside of an engine is, the better and longer it will run (other factors aside).

With regard to the "tight" vent system employed in BMWs: I'm pretty sure that the stalling when removing the oil cap is not caused by the PCV system. Every BMW I've looked at and paid any attention to has used a mass airflow meter to regulate mixture. These meters are always in the intake airflow after the air filter (usually immediately after) and before the throttle plate. Any leak in the airflow AFTER the mass airflow meter results in a miscalculation of the volume of air entering the engine (because air is entering the engine that is not flowing through the meter) and subsequently a failure to run properly, which usually manifests itself as a stall. This can be caused by a loose air intake hose clamp, for example. Now, if the pcv system uses the filtered intake airstream as its fresh air inlet point, as it almost certainly does, that means that by opening the oil cap you are creating a leak in the airflow in between the mass airflow meter and the throttle plate, thus providing engine management with false information and causing a stall.

All modern cars employ a PCV system, and no PCV system is "closed" in the sense that there is no inlet to the crankcase. A "closed" pcv system is one that routes its intake to the engine air intake system, breathing into the crankcase with filtered engine intake air; the filtered breather cap systems are referred to as "open."

For the question about the effectiveness of the Ford 6's PCV system, with the breather and valve both in the valve cover, I suggest taking a look at the FSM's illustration of the PCV system where it is described in detail. It shows that there is still a circulation of gases throughout the crankcase. This isn't really all that surprising given the turbulence caused by the crank and pistons whipping about in there, plus the constant pressurization occurring at the bottom end by blow-by. The gases are not stagnant in the lower half of the engine. Unplug your PCV valve from the valve cover and you'll likely see blowby gases issuing forth from the hole by their own pressure. These "dirty" gases don't have any trouble finding their way into the valve cover in spite of the vented breather at the other end.

One might also wonder why the PCV valve doesn't constitue a vacuum leak, or why the proper calibration of the valve matters (which can't be determined by the external appearance or shape of the valve), or why the valve's suction doesn't count as a leak in the case of the BMW with MAF sensor. As we know, the blow by gases enter the crancase and, if not vented, will pressurize it. Depending on how fast the engine is spinning, and how much throttle is applied, the rate of flow of blow by into the crankcase will vary. The PCV valve's job is to remove those gases at exactly the same rate that they enter (well, maybe a SLIGHTLY faster rate, I'm not sure) so that the crankcase is neither pressurized nor a vacuum. The valve can indeed operate as a check valve, but the spring isn't just for that purpose. It is calibrated, along the the shape of the moving pintle inside the valve casing, to create an orifice of just the right size to keep the crankcase unpressurized in any engine speed/load circumstance. At idle there is very little blow-by so the valve is almost shut; at WOT the valve is open as far as it can go. This makes sense; an orifice large enough to adequately vent an engine at WOT would cause a vacuum leak at idle. The tiny opening of the valve at idle does act as a vacuum leak, as far as I can tell, but the carburetor is designed for it and it is easily tuned to idle well. Note the difference in how your engine runs when, with no other modifications, you remove the PCV valve from the engine and plug off its vacuum port.

So, make sure that you are running the correct pcv valve. It can and will make a difference in crankcase ventilation and proper operation of your car.

I don't know whether major engine modifications like cams and carbs and heads and turbos mandate the use of a different pcv valve. I think they might. I avoid those questions by trusting the engineers and staying stock

 

[60s Refugee]

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