Firstly, there is no real vacuum to speak of in the intake manifold or plenum. Now there are a couple things happening at WOT. That's where the other half of the system comes into play. "Okay Ben, what happens when you're at WOT then?" I'm glad you asked. The manifold keeps a good vacuum on the crank case during idle and partial throttle conditions which amount to most of your driving. A little blow-by is pushed into the crank case but the intake manifold sucks it up and the oil separator in the valve cover keeps most of the oil from escaping the engine. That pathway is used around 90% of the time you drive the car. Here is another diagram showing the normal operation of the crank case ventilation system with half of the system greyed out: The oil that is collected drains back down onto the valve train and eventually returns to the oil pan. The more surface area the oil has to cling on to the more oil will separate from the air. Now the gasses have a bit of oil mixed in with it, so on it's way out of the valve cover it passes through a labyrinth of baffles designed to help condense the oil and collect the oil from the air. The gasses flow out of the PCV port and into the intake manifold where they are sucked back into the engine to get consumed and pushed out of the exhaust. The gasses are going to continue on through the small PCV (positive crank-case ventilation) port at the top-left corner of the valve cover which includes a one-way check valve so things can only flow out (and not in). Not a super great idea, but certainly efficient, practical, and good for the environment.Īll right if you've been paying attention we now have the blow-by gasses up and out of the crank case and in the valve cover now. Basically the engine is setup to consume it's own blow-by gasses. So the goal is to have a vacuum in the crank case, and this is accomplished on some vehicles with a dry sump oil system, but on regular vehicles the intake manifold is used for the source of vacuum. On the right the oily air can escape the crank case (in times where there is excessive pressure to evacuate) through the provision in the side of the block, up through the oil separator (catch can) leaving the crank case. This is the usual pathway for the air to take. On the left the air and oil can escape the crank case up along the timing chain portion of the block and into the valve cover. The amount of air and oil can be quite significant, so Nissan put in two important pathways for the pressure to escape from the crank case because at times there can be a lot of air to evacuate. I didn't show them everywhere in all the baffles, but you can imagine the same thing happening everywhere you see baffles. I even showed little oil droplets condensing out of the air as it goes up through the "catch can" oil separator. This air/oil mix is represented by the red arrows in the crank case and as oil gets removed throughout the system I've made the color shift towards blue. The air comes in contact with a lot of oil in the crank case and basically turns into a mix of air and oil (along with a little bit of gasoline and water). Since having a vacuum in the crank case is good and having pressure there is bad, we must get rid of the pressure somehow. Having a vacuum in the crank case is very good as it promotes ring seal and keeps windage losses (drag on the rotating crank caused by a cloud of oil in the crank case) to a minimum. This blow-by air is pressurized and will cause oil seals to fail and eventually will cause other major problems so it needs to be dealt with. The piston rings don't seal perfectly, so there is some air that gets by them and we call that blow-by. Now let me explain what's happening here. Here's the factory system (click image for larger display): I know people like images so I won't hold off any longer. It might also be nice to understand the history and evolution of the crank case ventilation system, so I'm going to start with the early SR20DE system and work up from there. To understand what you're doing when you modify or repair the factory crank case ventilation system you should know how the factory systems works first before diving into modifying it or fixing it. Keep reading to have all of your questions answered. Grab yourself a beverage of your choice, sit back and relax. If you're reading this you've likely asked a question about crank case ventilation, catch can placement, correct PCV hose routing, or similar. While this article will use a Nissan SR20 engine as a reference, the information found here can be applied to all motors.
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