Vacuum Mystery ?

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DustyEd

DustyEd

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'70 Duster 340 stock with original Carter AVS 4bbl. I have massive vacuum at the PCV on the driver's side, but none at the breather canister tube on the passenger's side. What's the cause and cure for this?
 
What are you calling "massive" and what are you calling "none?"

First, if you were to put a vacuum gauge onto the PVC valve (something which is mostly never done) you would read a high vacuum number at idle. THIS DOES NOT equate to a "massive" FLOW, because at idle, the piston in the valve is sucked up to restrict flow, thus NOT upsetting idle mixture.

THIS MEANS that at idle, there is LITTLE actual air FLOW VOLUME through the engine crankcase area cause by the PCV

Just about ANY blowby caused by even normal rings MAY nearly negate this, meaning, any vacuum at the breather is going to be not only slight pressure reading, but very little inflow, as well
 
Thanks for the reply. "Massive" as like a vacuum cleaner against a finger and "none" as like zero when laying a piece of paper over the breather canister hose end or the opening on the valve cover. I understand it should not be equal vacuum on the passenger side, like you mentioned. However, there should be enough to pull the paper to the opening. The owner's manual (1970) and a Glenn's Performance book (1971) both state there should be enough vacuum to pull the paper down, indicating the crankcase is properly getting vented. At idle the piston in the PCV is sucked up, thus opening the passage and creating air to be pulled out of the valve cover. I have tested my other two cars and they both have enough vacuum on the passenger side to pull the paper down. ('69 318 and '70 383)

More info....1) While idling, if I pull the PCV out of the rubber grommet and cover the opening with my finger, the car will die. 2) You mentioned rings, all 3 cars do not show any signs of worn rings.
 
67Dart273 said:
THIS DOES NOT equate to a "massive" FLOW, because at idle, the piston in the valve is sucked up to restrict flow, thus NOT upsetting idle mixture.
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I see what you're saying here, but the mystery remains in my thinking. Why is the 340 different?
 
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Are you sure there is not another breather opening, and how sure are you that this engine does not have more blowby than the factory ones beck then?

Factory performance engines, and I don't remember for sure about 340s, but 440 HP engines had a greatly reduced warranty and a VERY poor guarantee as to oil mileage. I don't even recall what that was, maybe 1 qt in 500 miles or something.
 
One side is a vent, to direct excess crankcase pressure/oil vapor ( blow-by that increases with rpm as vacuum decreases with more throttle) up to the carb to be burned, no vac nec.
Paper is drawn by vacuum from other side .

EDIT
 
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OK, with the points you all mentioned above and a private phone discussion with a well experienced Mopar fellow, also mentioning the rings, I'm rethinking the RINGS could be the answer. Even though, there has been no other indications of worn rings.

Thanks to everyone. Any other comments still welcome.
 
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Do you have a concern or just curious ?
Blow-by exists to some degree in all engines .
By your description, you have normal engine conditions .
Ring issues will show up on your spark plugs and from your tailpipes .
 
A simple test for blowby [ caused by worn rings/bores ] is to remove the PCV hose from the carb & plug the carb fitting. Warm up the engine. A worn engine will have a lot of fumes/smoke exiting the valve cover breather. While not a definitive test, it does give some idea of wear. No fumes would be good.
 
Maybe you have another source of incoming air. Like an intake manifold gasket leak, or a carb base gasket. That would explain why you have no draw on the other valve cover.
 
It's not uncommon for the engine to idle way down or even die when covering the PCV valve. It is in essence a controlled vacuum leak in normal operation. If the engine is tuned for the PCV, and I'm sure it is, if you disconnect it and plug it, it is entirely possible to get your result, since the resulting "plug" of your finger or thumb would stop the "controlled leak" completely and probably pull the idle down more than when the PCV is plugged in and working. It's also entirely possible it might cause the engine to stall. Sounds to me like the PCV system is operating normally.
 
Also remember this: When you are OFF idle, the pcv orifice gets LARGER as the piston inside moves. If you are not getting blowby indications, or excessive oil use, or blue when shifting/ "on it" or letting off out the tailpipe, I simply would put this to bed
 
67Dart273 said:
Also remember this: When you are OFF idle, the pcv orifice gets LARGER as the piston inside moves. If you are not getting blowby indications, or excessive oil use, or blue when shifting/ "on it" or letting off out the tailpipe, I simply would put this to bed
Click to expand...
A nap sounds mighty good about now.
 
inertia said:
Do you have a concern or just curious ?
Blow-by exists to some degree in all engines .
By your description, you have normal engine conditions .
Ring issues will show up on your spark plugs and from your tailpipes .
Click to expand...
My concern was having proper internal air flow/ ventilation in order to avoid internal pressure.
 
TT5.9mag said:
Maybe you have another source of incoming air. Like an intake manifold gasket leak, or a carb base gasket. That would explain why you have no draw on the other valve cover.
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Good points. Last night I spray around the intake while idling....no response. Then, did same on carb base.....got a slight response in the front. Found out the bolts were barely tight. I tightened to 200 inch pounds. However, no change at the air breather (pass. side)......what the ? Should I change out the gasket just in case?
 
DustyEd said:
Should I change out the gasket just in case?
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Not if you tightened it up and it quit leaking. Move on to the next thing. Don’t rule out intake manifold gaskets. Or how bout the oil dipstick tube?
 
i guess you've run out of jobs to do on the 3 cars and like to tinker? hence the looking for problems that aren't there approach? if there's no smoke or high oil consumption and it runs fine then it usually is.
neil.
 
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