Blow-By and How Much it is Reduced by a PCV


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By '59Edsel - 11 Years Ago
I recently had vacuum issues. My PCV valve was too large and allowing too much of a vacuum leak. So I replaced it. My new PCV valve seems to be sucking less blow-by. I see some blow-by coming through my oil breather cap. Do you think the valve is worth having now? I made a video:


While I'm driving, especially when the engine is working its hardest, I notice that after I park and check under the hood that the oil breather has sprayed a bit of oil over my intake manifold, valve cover, and air filter housing. Does this mean anything significant?

By the way, just curious what you guys think, I installed a better branded pcv, an ac delco, and my vacuum went down a bit. In order to bring it back up I sleeved the existing 5/16 hose with a smaller hose, and the put a vacuum restrictor into that. Still pulls a vacuum and bumped up the vacuum to about 18 inches.
By Ted - 11 Years Ago

Assuming the engine is in good condition, a PCV valve will remove all the excess blow-by that's being generated under normal driving conditions.  Your vacuum reading looks good so there doesn't appear to be much wrong with your engine.  As long as you can feel a suction at the end of the PCV valve when it’s pulled from the engine, you should be okay.  The correct PCV valve should not require any restrictors in the hose though so that’s something for you to look at.  An option for any fumes that are still coming from the breather is to use a breather with a hose that connects to the air cleaner assembly which will then allow any excess pressure from the oil breather itself to be vented to the air cleaner.
By Hoosier Hurricane - 11 Years Ago
A PCV valve is an orificed check valve.  At idle, with high vacuum, the flow through the PCV is small.  At lower vacuum levels, when driving the car, the check valve unseats and allows a larger vacuum path for the blowby.  A restriction in the hose to raise your idle vacuum reading defeats the purpose of the PCV valve.
By '59Edsel - 11 Years Ago
Got it, i will switch my PCV to a model I don't need a restrictor on. Does anyone have any thoughts on the oil spewing out of my breather under high RPMs?
By Hoosier Hurricane - 11 Years Ago
Not enough breathing because of the restricted hose?
By miker - 11 Years Ago
I think John and Ted are right, not enough flow thru the valve will force the balance of pressure (blow by) out the fill tube. Not sure this is the right one, but this is an example of the oil fill cap for a closed system. Whether it's what you want to do is dependent on your air cleaner, and if you want to modify it. I run all my motors with closed systems because I don't like the oil smell.

http://www.ebay.com/itm/Cal-Custom-Oil-Filler-Cap-/111226113552?pt=Motors_Car_Truck_Parts_Accessories&hash=item19e597ce10&vxp=mtr