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PCV issue

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Old 07-24-2012, 04:51 PM
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PCV issue

What does the PCV valve really do? Seeing that the road draft tube is always pulling fumes out couldnt you just have a tube coming out the back of the valley pan that has vacuum hooked to it with baffling so it wont pull oil also? I ask because I got a valley pan with a PCV valve on the back with the original baffling but as you can see in the picture there is no "valve" in the middle but just a open tube. Thanks

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Old 07-24-2012, 05:21 PM
85e150six4mtod 85e150six4mtod is offline
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PCV uses vacuum to circulate air through the crankcase, sucking the fumes into the intake to be burned.

A road draft tube works as a vent, but only really works when you are moving and creating that slight vacuum by having the air move over the end of the tube.

For either system, you need a source of clean air to go into the crankcase. This is usually a breather with a filter in it, or a tube from inside the aircleaner housing on OEM PCV equipped vehicles.

My dad had a '53 GMC military truck that used a PCV set up as part of sealing the crankcase so the truck could run through deep water. (54" iirc) There was a snorkel into the cab that had to be opened.

Note the "P" stands for "Positive", and the PCV works at all times, not just when you're moving.

Besides, the road tube drools on the driveway don't it?
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Old 07-24-2012, 05:34 PM
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Got it but do I need the 'valve' portion or can I use the old PCV that has the guts removed that is pictured? My question is what does the valve accoplish since there will be vacuum from the carb to pull the fumes out the crankcase. Why does there have to be a valve that opens and shuts?
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Old 07-24-2012, 05:53 PM
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It is my understanding the the PCV valve, as found in later stuff, limits the amount of vacuum, and also functions as an oil stop if it starts to suck oil instead of just fumes. An open vacuum into the crankcase would cause some problems methinks.
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Old 07-25-2012, 06:32 AM
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Without the valve you will have a truck with a vacuum leak and this will impact the other systems that require vacuum. Bottom line you need the valve if you want to use vacuum instead of the engine vent tube.

Looks like you can fit a rubber grommet in the hole that will accept a PCV valve in the "body" you are holding. I did something similar. I removed the tube, plugged the hole with a grommet that allowed me to install an off the shelf PCV.

Make sure you have a vented (with filter) oil fill cap to allow fresh air into the crank case.
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Old 07-25-2012, 04:34 PM
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Got it, thank you.
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Old 07-26-2012, 07:07 AM
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Also a lesser know function of the PCV valve is to keep gas vapors from entering the crankcase. I had a friend once who designed his own system on a 58 Chevy 283. He didn't use a PCV valve. Once the car backfired a little when he was starting it and it blew the valve covers off and the oil pan held a couple extra quarts after that.
Also the valve is designed to route crankcase vapors under normal driving. The side of the system with the filter where it can draw fresh air is also designed to let excess vapors escape under hard acceleration.
Then cars with a lot of blowby end up with oil in the air cleaner where the vent hose goes.

You need a proper functioning valve if you install one. If you think you need more flow, get one from a 460.
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