Are valve cover breathers a big vacuum leak?

Is it ok to just eliminate the PCV valve and tubing at the rear of the intake manifold? I could leave the tube going from the filler neck to the intake tube so that there is still ventilation, but I have oil coming up the PCV valve going into the manifold and fowling my two rear plugs. I would rather not have to put an in line oil seperator in there to fix this problem, but I have to do something. Will it still pass emissions as long as they don't realize the PCV is gone?

A breather alone is not a vacuum leak per se. In other words you should not have issue using breathers and plugging the PCV.

Most of the issue with mixing the two methods comes from unmetered air (air being pulled in through a breather and redistributed for reburn through the PCV. The MAF never knew this air existed).

Run a breather in each cover and plug the pcv like said above or just run the stock pcv set up w/out breathers. Do not run a pcv and breather together as this is where you will run into problems. Its one way or the other not both like hissin said.

you can also run a breather in the back of the intake where the pcv valve goes. its better to have too much ventilation than not enough.

I don't want to pull my VC off. I am really thinking about just running a breather on my oil fill. Could I put a breather on my lower intake instead of the pcv valve or would that still be a leak?

FWIW when i take off my oil cap, i see NO difference in idle quality or vacuum. i dont see how a breather would be different

From what I've heard the only down fall of running 2 breathers is that you can get a light misting of oil that comes out through the breathers.

I think the only question that hasn't been answered yet is is one breather sufficient? With our manifold design, it's hard to get a breather on the driver side cover.

Anyone?

One PCV valve isn't even adequate on modded motors, so I'd want to do two closed breathers.

Hissin, could you elaborate? Is there more crankcase pressure in a modded motor, overwhelming the design of the stock PCV?

Along those lines, is the reason that a breather does not present a vacuum issue that it generally only expells (assuming the PCV connection is removed) air due to blowby?

Just curious. I had AFM work on and tune my TFS top end kit, and Danny recommended a breather, but I can't recall whether he plugged the PCV. Now I'm curious, but I'll never know, because I cracked my block and had a shop tear it down (a whole 'nother story).

Brad, a modded motor can introduce a couple of added factors. When a leakdown test is done on a fresh mill, it can often display notable (note I didnt say excessive, but notable) blowby.

On modded motors which turn higher RPMs, the air displaced by piston movement can create extra pressure.

When tearing down a motor, seeing a ton of non-solvable grime indicates that there was a bottleneck. The trapped vapors can make a corrosive goo that's tough to remove.

If a PCV system is overwhlelmed, the fluid (air, contaminates, etc) moves at a faster clip, which can allow it to transfer more liquid volume to the intake. Reconfiguring the system to flow more volume at lesser pressures can help. This generally requires multiple points of egressing crankcase vapors. Making shorter runs of plumbing in general can help too (the stock PCV plumbing is kinda long).


I don't think I followed the question in the second paragraph (my fault, not yours). There is indeed less expulsion with a breather (since it passively vents, rather than being connected to intake manifold, which actively sucks the vapors out). Often, just the breather placement is what helps (the vapors have an easier, less convoluted way out).

With a breather set-up, the MAF isnt affected by umetered air. If the PCV and breathers are mixed (remember, the little line from the oil filler to the air-inlet is part of the PCV), then the unmetered air (sometimes called a vac leak) issue arises. On a SD car, it wouldn't matter (it's just an issue with accountability of air mass).

if your questions weren't answered here or would just like some more info check this out:
http://www.sn95forums.com/forum/index.php?PHPSESSID=9450f5ac26de94bd009f8ac6cac7d8a8&topic=9567.0