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Old 05-04-2004, 06:52 PM
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Default Is a PCV valve necessary?

A friend of mine came by today to shoot the breeze and drink a cold drink. While he was here, we got around to discussing the blow-by on his 304. The engine is very worn. A compression check shows all cylinders below 90. The engine exhaust had very little smoke and what was there appeared to be from an over rich condition. His carb was covered with oil inside and out from the blow-by coming out of the oil fill cap and into the air filter housing. He had no idea how a PCV valve worked as the vacuum line from the carb had a Tee on it with another vacuum line going to a vacuum port on the Performer intake. Two sources of vacuum keeping the PCV valve closed or nearly closed longer than it should be. After we removed the tee and capped off the other vacuum port, there was still a lot of blow-by going into the air filter housing. We installed a new PCV valve that I had and the blow-by was the same. We discussed putting another PCV valve in the valve cover and using two, but instead decided to take the PCV valve completely out of the system and just stick the hose from the carb base into the grommet on the back of the intake. The blow-by from the oil fill tube ceased and there is now a slight vacuum there as the engineers intended. The engine RPMís increased by about 3 or 4 hundred. I adjusted the idle down and adjusted the low speed jets on the Holley 600. The engine runs better than it has in a long time. There was some blue smoke coming from the left exhaust but it stopped after a few minutes of run time.
Now, finally, my question. With as much blow-by as this engine has, what are the concerns of not using a PCV valve? My only concern is an engine backfire causing crankcase gas to explode. Iím not sure if this could happen or not.
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Old 05-04-2004, 11:10 PM
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Default Re: Is a PCV valve necessary?

I'm wondering how the thing is idling at all. This must be one rich running engine to allow an unregulated vacuum leak to not cause it to stall. No normal engine can do this. The volume of air bypassing the venturi (and therefore "unfueled") would not support idle ..at least a smooth one (loping, erratic, etc. maybe). The PCV valve is, in effect, a regulated manifold leak. Sufficient vacuum opens it ....as it bleeds off too much vacuum ...it closes. That's why you hear them continually flutter at idle. It's cycling with the state of manifold vacuum.

This engine, or so you indicate, is null and void. Since your friend is getting oil in the air cleaner ..........you've got to have a hose to inside the air cleaner ..and not just a "breather cap". Take this hose off of the air cleaner and hang it in a jug inside the engine bay ..empty as needed. If you want to get fancy . use these.
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Old 05-05-2004, 06:54 AM
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Default Re: Is a PCV valve necessary?

I don't understand how this engine is idling either, unless there's so much oil mist and unburned combustion gasses in the blowby that it's compensating for the lack of added gasoline.

I'd say that whatever works is a good thing.

FYI I attempted to cure a problem by making a manifold that put 2 PCV valves in parallel. It worked fine but didn't cure the problem.
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Old 05-05-2004, 06:58 AM
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Default Re: Is a PCV valve necessary?

Thanks for the reply GA. I agree with you about the unregulated vacuum leak. Before we removed the PCV valve, pulling the hose off the carb caused the engine to stall. Now if we pull the hose out of the intake or carb, the engine still stalls. Like I said in the earlier post, the engine is performing better than it has in a long time, maybe better than it ever has since my friend has owned it. It idles fine and has more power from idle up.
My concern is still what problem not having a PCV valve will cause. I keep thinking that solving his problem with oil everywhere canít be as easy as removing the PCV valve until the new engine is ready to install.
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Old 05-05-2004, 07:26 AM
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Default Re: Is a PCV valve necessary?

Well, getting to your final question....I don't see any "risks" of not running a PCV valve. Your setup just operates full time while the PCV valve would regulate it. My 78 BMW 320i didn't have one. The crank case was under full manifold vacuum ..if you opened the oil cap on the valve cover ....it stalled. [img]images/graemlins/blush.gif[/img] I would suggest that either the hose you're using is half clogged ..or the ultimate pathway to incoming air is restricted in this engine (sludge, etc.) making it a long narrow path ...and therefore "simulates" a "fixed" manifold leak that you can adjust for (much like a metered orifice except larger). The oil vapor is better going through the combustion process instead of just being puked all over everything (or so our contemporary blowby management techniques would lead us to believe).

Since this engine is shot (beyond just tired) ..virtually anything that you do to make it functional is okay (heavier oil, hotter range plugs, etc.). Your friend has nothing to lose.

So ...accept a pat on the back from your friend ...you've enhanced his "drivablility" and allowed him to enjoy his engine a little more for a few more miles. Get him to buy the next round. [img]images/graemlins/40BEER.gif[/img]
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Old 05-05-2004, 10:20 AM
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Default Re: Is a PCV valve necessary?

You'll find out why PCV Valves are used when the valve covers blow up through the hood, or the oil pan blows off the bottom.

A PCV Valve is two things -- a restriction to limit the amount of air pulled out of the crankcase to keep fumes from collecting in the crankcase, thus leaning the mixture -- it's a controlled vacuum leak, and it's a one-way valve to prevent explosions in a backfire.

Normally just sitting the valve is open. Normally hooked up properly the valve is still open - it's spring loaded with a light spring. The valve is always open unless a backfire occurs, then the backfire's pressure in the intake manifold -- air going backwards in the valve - shuts the valve off to prevent fire from getting in the crankcase. Simply blow through one gently, then harder to verify.

Sometimes engines are rebuilt when they aren't needed - not routing a PCV Valve properly is sometimes the cause.

The PCV SYSTEM is important. It creates a draft through the engine, sucking all the unburned gasses out. Fresh clean air MUST get in one side of the engine - that's usually a hose to inside the airfilter element, or has it's own filter to keep out the dust.

The other side is the PCV Valve line. It hooks to the intake manifold BELOW the carb - sometimes it's at the carb base, sometimes at a monifold Tee. Never above the carb or into the air cleaner. The more centralized the location the better. The other end is hooked into the crancase - usually through a valve cover grommet.

Be careful - where it hooks is suction - the hole in the valve cover must have a baffle inside to prevent oil from splashing in the hose. Many times that oil splash is diagnosed as blowby. Remember the rockers are throwing lots of small oil droplets off, those droplets can easily get sucked up in the hose.

The vacuum in the manifold draws air and fumes through that hose, through the open PCV Valve, then into the manifold, not into the carb.

A popular misconception is blowby richens it -- oil from the blowby does not burn - oil itself won't burn well enough to change the mixture at all, but it will foul plugs.
If the blowby is so bad it's got enough gasoline in it to be flammable, there's other major problems anyway. In those cases the PCV SYSTEM is doubly important to prevent an explosion!

Yes, the PCV System is a small vacuum leak, but it's constant, so the carb compensates for it.

BMW's and the like don't use a valve in the PCV line, but that hose is much smaller and doesn't carry the fire in a backfire as well. But when they do blow - and they do - BMW - or Mercedes etc., makes lots of money!

When you hear a PCV Valve making noise it's because the vacuum in the manifold (if it's really hooked to manifold vacuum) is fluttering/pulsating for some reason. It's a good indication of something else wrong, ie. valves, overlap, low compression, exhaust restriction etc.

Yes, 2 PCV Valves can be run in parallel. Just make sure there's baffles on both and a fresh clean air source.

90 lbs compression may not be all that bad -- as long as all cylinders are about the same. It could be the method used to take the compression -- engine warm, battery fully charged, all plugs out, throttle blocked open, accurate screw-in gauge, and exactly the same number of pulses on each cylinder - 3 to 4.
Any procedure other than that is fooling one's self.

Rebuilders love misdiagnosis!
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Old 05-05-2004, 10:57 AM
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Default Re: Is a PCV valve necessary?

Thanks JIM LOU for the reply. I am going to call my friend and ask him to pickup an air filter on his way over today. The old air filter is a 10 inch that has had a lot of oil mist sprayed on it from the blow-by.
I can see the scenario now:
Install new filter.
More air through top of carb.
The engine runs like crap because of the amount of air being pulled into the vacuum port.
Install the old filter.
Drink some cold ones while trying to figure out how to manage blow-by when a new filter has to be installed.
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Old 05-05-2004, 02:24 PM
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Default Re: Is a PCV valve necessary?

RRich - thanks for the detailed reply. I knew there had to be another reason to use a PCV valve. I had thought about the possiblity of an explosion, just wasn't sure it could really happen.
It was my understanding before reading your post that the spring kept the valve open until manifold vacuum over rode the spring pressure, which was only supposed to happen at low RPM's. As the RPM's increased, manifold vacuum decreased and the valve opened completely. It doesn't really matter in our case, just thought I would tell you that I learned something today.
We removed the air filter a little while ago and there was only a very very small change in the engine. So, we still don't know why the engine is performing so well without the PCV valve.


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Old 05-05-2004, 05:22 PM
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Default Re: Is a PCV valve necessary?

Removing an air filter should NOT make any change -- if the filter's big enough and clean enough. It's supposed to CLEAN the air, not restrict it.

The extra added air from the open line without the Valve's restriction is compensated by your idle adjustment. If you didn't adjust it when you made the change, it was running waaaay too rich anyway.
And, there's a possibility that the crankcase has lots of fuel in it already if it was running bad before. That fuel is now being drawn out of the crankcase by the hose -- but eventually all that fuel will be "used up" and then it'll be lean.
That's why when adjusting a carb properly it should be done after an oil change, or at least unplug the PCV Valve from the valve cover and let it dangle - sucking in fresh air. Then you adjust it right. When you plug it in the valve will evacutae the crankcase and then it'll be running like it should.
We've all experience adjusting a carb after a tune-up - a few days later the customer comes back with it running terrible. We adjust it again, no more problem. That's because we adjusted it while the PCV System was artificially enrichening it - till the oil got clean.

Oil on the filter can plug it, making it run rich. But oil itself doesn't burn like gasoline - try it, dribble a little down a lean engine to try to richen it up.

And, try to keep the manifold end as centrally located as possible. If it's at one end, it leans those cylinders nearby, but the other ones don't get the extra air - creating an imbalance.

Nope, only closes on a backfire. Blow through it - it's a one way valve.

Yes, they do explode!


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Old 05-05-2004, 10:33 PM
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Default Re: Is a PCV valve necessary?

Thanks RRich. I will try to decipher what you wrote and see how it applies to our situation.
I think you are saying that adjusting the idle and the low speed jets compensated for the extra air drawn into the mixture through the vacuum port. If true, that would explain why the engine still runs without a PCV valve. Why it runs so much better without the valve is still a mystery to me.
It seems obvious to me that there was, and still is, a serious problem with the fuel air mixture going into the engine. Removing the PCV valve masked the real problem, which is still to be found.
More research tomorrow.
Thanks again for your help.
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