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I would think it would lead to an excess of blow by, the upper part of the engine is surrounded by a mist of oil and it goes into the clean air intake of the PCV system. The valve cover has diffusers build into the top to separate the oil from the air before it goes into the PCV valve. The clean air intake usually does not have a baffle when entering.
The PCV system in your car is designed to burn the blow by gasses that make it into the crankcase. When your car burns gasoline in the combustion chamber, some of the combustion gasses make it past the piston rings and into the crank case. Now, old cars had a pipe that went from higher in the crank case and just vented it to the atmosphere. This was called a draft pipe. When the car was moving, because of where the tube was located, the negative pressure from around the tube drew the gasses out of the crankcase. Venting combustion gasses to the atmosphere isn’t cool, so manufacturers designed a system to reburn these gasses
The PCV valve has a little spring in it to resist high engine vacuum during idle, deceleration, and cruising. The spring is there to prevent too much oil vapor from being sucked up into the intake and burned. If the spring fails, the engine will suck up too much oil and oil vapor and cause a low oil problem. The spring can occasionally gum up and seize as well, but this is more rare because it’s pretty well lubricated by the oil vapor.
When a PCV valve clogs, it can lead to a whole bunch of other problems. When it is clogged, there is no place for the pressure inside the crankcase to go. This usually leads to the blowing out of seals or dipsticks. This can be much more costly when a seal blows out, as you have to replace it, and it can lead to a sudden loss of large amounts of oil, rather than a slow consumption with a PCV valve that is stuck open.
I hope that explains it. Let me know if you have any more questions.
Hey Hockeyclark. Really thank for your very detailed explanation. I however still need your explanation about little thing.
Actually, what I understand about the PCV valve is it “communicates” between the crankcase and the intake manifold. I also understand this is “one way” flow (air from the crankcase to the intake manifold). The spring inside of pcv valve blocks only the air back to the crankcase. So when the vacuum is high inside of intake manifold (idle, deceleration ) , I don’t see anything that block the air from crankcase to the intake manifold. Am I missing something ?
The PCV valve is really an “anti-backfire” valve. It’s purpose is to prevent a backfire in the intake from entering the sump.
If intake backfires were impossible, the entire PCV system would consist of nothing more than a hose running from the sump to the intake, no valve required.
Thank Evil-I for the explanation. I have pretty good idea how the PCV works. But what is still bugging me is I try to understand how a bad PCV valve can increase engine oil consumption.
If the PCV valve is stuck open, the engine will use its full vacuum to suck oil out of the motor and burn it. Think of it like holding a partially clogged vacuum hose near some water. When the hose is partially closed (the PCV valve working properly), the vacuum isn’t strong enough to suck the water up. When you unclog it (the PCV going stuck open), it starts sucking the water up. The PCV valve is there to limit how much vapor is sucked from the crank case. When it’s stuck open, the full vacuum of the engine is exposed to the crankcase, or the valve cover, where it usually comes from. The oil is close to the hole, and so when the suction is too strong, oil goes in the PCV hose and is consumed by the motor. It should be noted that sometimes you will not notice the blue smoke in the exhaust, as it will not be thick enough, though you will still be consuming oil.
You can’t actuate the valve that way. Humans can only pull about 2.7 inches of vacuum with their lungs, no matter if sucking/blowing. An engine pulls about 20 at idle. Any PCV valve is going to need more than 2.7 inches of vacuum to function. You could try blowing into the intake side or sucking from the crank case side, that would probably be the only way to actuate the valve. You would be actuating it in its anti-backfire capacity as Evil-i explained. You were going about it backwards, I think.
And we don’t judge anyone here at the ETCG forum.
Ok. It makes sense. And thanks again for your very thoughtful and detailed explanations.
The reason why I need to know how the PCV valve works because my car, a 2000 Honda civic, burns about 1qt/3k miles of oil if I drive almost exclusively in the hwy. But it burns almost nothing if it was uniquely city driving. This is why I suspect the PCV valve has to do with this odd oil consumption pattern.
you can like check the PCV vulve if its good or bad my shaking it up and down it should be easy and raddle a lot when you shake it up and down… you can like try and blow both ways throw it one way it will be easy and other will be hard… I personlly take my out and clean it with starter fuild each oil change and once a year I replace it… if it sounds good or bad… I replace it… they are only like $3…
to add PCV being your main and only cuase to you burning 1qt of oil seams odd to burn that much… uslly its the rings or vulve’s, seats,or even the head gasket….
to me it sounds like you have something else wrong too…
if you want to check your theiry out that the PCV is cuasing the problem… check the spark pluggs they all should be the same and all should have the same amount of oil on them…. and do a compression test… all the cylinders should read the same or very close after you replace teh PCV….Thanks Rob781 for the advices.
The spark plugs are fine and the compression are good as well.
https://www.ericthecarguy.com/kunena/8-Service-and-Repair-Questions-Answered-Here/64024-what-do-you-think-about-these-plugsHonestly, I don’t think you can blame high oil consumption on the PCV valve. Years ago, when the valveless “road draft” PCV system was used, the sump was being vented at all times, and excess oil loss was never an issue.
Ironically, if the valve is permanently shut, it’ll result in oil leaks, because the sump gets pressurized and will force oil out wherever it can. That’s why crankcase ventilation was adopted in the first place.
The PCV system doesn’t suck liquid oil out of the sump, so let’s put that issue to bed. The oil the PCV valve sees is oil mist that is entrained in the crankcase vapors. Usually, there are baffles somewhere in the PCV air path that will strip the oil out of the passing vapors, allow it to coalesce, and allow it to drain back into the sump. If you are getting so much oil mist in the crankcase vapors that it overcomes the baffles’ ability to cope, you’re looking at an advanced case of blowby, where so much compression and combustion pressure gets past the rings that the crankcase environment is violently turbulent and it experiences repeated sharp pressure increases, which can churn up the oil in the sump by an alarming amount. And let’s not forget that the pistons pumping up and down, and the spinning crankshaft also contribute a lot to the hurricane in the sump. The PCV valve itself cannot cause blowby. It deals with blowby when it’s present.
If you are experiencing high oil consumption, and the PCV system is seeing a lot oil, I would first make sure that you aren’t over-filling the sump, allowing the crank to churn up the liquid oil. If that isn’t the problem, I’d say you are experiencing some mechanical defect, like poor piston ring sealing and/or cylinder bore wear.
@Evil-I, I agree most of your assessment regarding the PCV valve and oil consumption. But if you do a search regarding the causes of oil consumption, the pcv valve is very often mentioned as the cause of oil consumption.
I’m not worry about my engine burning oil. Because 1qrt / 3k miles it’s not a really a big deal. But It’s just weird that my car burns oil only in the hwy driving and I was also curious about how people come to conclusion that bad pcv valve can cause the increasing oil consumption. that’s all.
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