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Old 08-28-2005, 18:38   #1 (permalink)
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How do you choose the right PCV ?

My motor idles MUCH better if I pull the PCV out of the rocker cover and block it with my thumb. I thought the PCV is supposed to be closed during idle, then open during mid throttle? All mine’s doing is creating a huge vacuum leak at idle.

Previous threads have clearly indicated the need to always use a PCV, but how do I know if the one I have is working correctly or suits my motor?

What steps do I need to follow in order to select the correct PCV?

For starters, I’m guessing that I connect a vacuum gauge to the manifold, then block the PCV with my thumb and take a vacuum reading at idle. Do I then choose a PCV with a reading above or below this figure?
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Old 08-28-2005, 18:41   #2 (permalink)
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Re: How do you choose the right PCV ?

I wasn't aware PCV's had a rating?
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Old 08-28-2005, 19:15   #3 (permalink)
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Re: How do you choose the right PCV ?

? What is a PCV ? still trying to learn more, from you two, as you seem to know alot of stuff.
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Old 08-28-2005, 20:25   #4 (permalink)
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Re: How do you choose the right PCV ?

Quote:
Originally Posted by brenx
I wasn't aware PCV's had a rating?
I'm probably wrong, but I thought I saw a stamped number on the round metal part that goes up and down.

When I was in the auto parts shop, I saw a few of them on the wall with different numbers, though they all looked the same externally.
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Old 08-28-2005, 20:27   #5 (permalink)
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Re: How do you choose the right PCV ?

If you do replace yours make sure it's a metal one. The plastic ones fatigue and can do damage.
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Old 08-28-2005, 20:59   #6 (permalink)
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Re: How do you choose the right PCV ?

If you are running a big cam and don't have very much vacuum at idle, I'd choose a pcv valve from a factory car that had limited vacuum. When I was running a pcv valve, I got one from a 1970 Boss 302. The part number was the same for that car and some of the hot 427 powered cars from the 60's.

"? What is a PCV ? still trying to learn more, from you two, as you seem to know alot of stuff."

The pcv, positive crankcase ventilation, valve is used to help pull out gasses and positive pressure out of your engine via the valve covers. It pulls it out by vacuum (on the intake manifold or carburetor) created by the intake charge rushing into your engine, same as engine vacuum, or manifold vacuum, whatever you know it as. It is a valve so in-case the engine would backfire up through the intake, it can't get back into the valve cover where it could cause an explosion from the unburnt gasses. By not running a pcv valve, the engine will have excess positive pressure in the crankcase, the rings won't seal as good and oil will be forced out of gaskets and seals more easily. Hope that helps.
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Old 08-28-2005, 22:01   #7 (permalink)
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Re: How do you choose the right PCV ?

The motor is a stocker and the PCV is sucking strongly at idle, thus causing a large vacuum leak.

Is the PCV supposed to be sucking at idle?
I'm finding lots of conflicting information on the Internet.
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Old 08-28-2005, 22:46   #8 (permalink)
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Re: How do you choose the right PCV ?

The following link may answer my question, and indicates that the PCV should be sucking all the time, except when the motor is off or backfires. The article also has a useful diagram.

http://autorepair.about.com/library/faqs/bl616h.htm
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Old 08-28-2005, 23:01   #9 (permalink)
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Re: How do you choose the right PCV ?

on a (kinda) related subject, could the PCV be replaced by a catch can or breather or something? Anyone know if they're legal for LPG??
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Old 08-29-2005, 01:35   #10 (permalink)
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Re: How do you choose the right PCV ?

Quote:
Originally Posted by Rhett
My motor idles MUCH better if I pull the PCV out of the rocker cover and block it with my thumb. I thought the PCV is supposed to be closed during idle, then open during mid throttle? All mine’s doing is creating a huge vacuum leak at idle.

Previous threads have clearly indicated the need to always use a PCV, but how do I know if the one I have is working correctly or suits my motor?

What steps do I need to follow in order to select the correct PCV?

For starters, I’m guessing that I connect a vacuum gauge to the manifold, then block the PCV with my thumb and take a vacuum reading at idle. Do I then choose a PCV with a reading above or below this figure?
If you take it out and shake it, it should rattle. They can "gum up" inside after a while, and then the little bit inside won't seal properly.

A PCV should seal at idle, and allow crankcase pressure through when you are cruising along the road, when the engine is more able to burn it.

You can try cleaning it (soak in a solvent), or fit a new one. But, it definitely should seal at idle.

Also, just checked with a Ford (factory) workshop manual, and it would seem that they agree.

"A malfunctioning positive crankcase ventiliation system may be indicated by loping or rough engine idle"- Ford XD workshop manual
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