Civic VX PCV valve, or lack there of. - Page 2 - Fuelly Forums

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Old 02-25-2009, 09:40 PM   #11
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In previous discussions, I was told that the proper VX PCV "valve" was just an elbow with a restrictor orifice in it. Normal PCV valves will let air flow fairly freely when the pressure difference/flow is low, but a spring-loaded plunger heavily restricts flow (but doesn't stop it) if the pressure difference increases enough.
My only guess is that the VX's cruising conditions resulted in either too much, or too little ventilation in the crank case when paired with a typical PCV valve. Beyond that, I don't see why a typical valve couldn't be used.
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Old 02-27-2009, 03:22 AM   #12
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Can we just call you "Bruce" to avoid any confusion?

Anyway - the PCV valve restricts flow from the crankcase to the intake manifold when the manifold vacuum is high - closed throttle. When the throttle is opened and manifold vacuum decreases, the PCV valve opens because the plunger inside of it is not being pulled down by vacuum as strongly. The point is that at a larger throttle opening, the engine is consuming more fuel, more blow by gets past the piston rings, and the crankcase gases need to be consumed.

The VX has a fixed orifice PCV valve which Toyota has also used in earlier PFI engines.

The primary drawback to a fixed orifice PCV valve is that the line between the crankcase and the intake manifold cannot be closed off. The spring loaded PCV valve completely closes off the vacuum line when the engine is turned off or when there is positive pressure in the intake manifold. This prevents igniting the blowby gases in the crankcase if a backfire occurs in the intake manifold.

The flip side of a backfire into the intake manifold with a spring loaded PCV closing off the crankcase is that the intake manifold could explode! - this has happened with GM composite manifolds appararently.
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Old 02-27-2009, 06:29 AM   #13
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Intake manifold explosions shouldn't be an issue with port/multi-point fuel injection engines (that is, almost every fuel injected Honda engine). Unlike a throttle body injection system, there is at most a few cubic inches of air/fuel mixture in the intake manifold at any given time. Yes, it could backfire, but the results would be less than impressive.
As for on-throttle blow-by, there's an unrestricted breather tube from the valve cover to the air intake tube before the throttle body. When cruising (high manifold vacuum), the little bit of air flow that makes it past the PCV valve keeps a constant flow of fresh air into the crank case through that breather. When accelerating (low vacuum) any excessive volume of blow-by gasses would just make the air flow through that valve cover breather tube reverse, injecting crankcase gasses in upstream of the throttle body.
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