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Discussion Starter #1 (Edited)
'96, 3.6, I have negative air pressure in the valve covers. At the back of the valve cover on the passenger's side is a breather tube that connects to the large air intake. If I remove the tube and put my thumb over the hole in the valve cover, there is a vacuum. This seems backwards to me. Usually the air under the valve covers is positively pressured due to positive crankcase pressure? What's up?
 

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96 3.6 dual carb? wtf?
 

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rix444 said:
96, 3.6, dual carb. I have negative air pressure in the valve covers. At the back of the valve cover on the passenger's side is a breather tube that connects to the large air intake. If I remove the tube and put my thumb over the hole in the valve cover, there is a vacuum. This seems backwards to me. Usually the air under the valve covers is positively pressured due to positive crankcase pressure? What's up?
If i remember correctly it's a smog reg tube to recycle used gasses back into the engine to cut down on emissions.
but then again i'm tired and mildly drunk so i may have no idea what im talking about.
 

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Discussion Starter #4
I agree, it's probably a tube to recirculate the bad gasses, so why is the pressure backwards? Right now it's sucking clean air into the engine block.
 

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rix444 said:
'96, 3.6, I have negative air pressure in the valve covers. At the back of the valve cover on the passenger's side is a breather tube that connects to the large air intake. If I remove the tube and put my thumb over the hole in the valve cover, there is a vacuum. This seems backwards to me. Usually the air under the valve covers is positively pressured due to positive crankcase pressure? What's up?
Things haven't worked that way for a long time! PCV systems are designed to tap into the vacuum from the intake to pull the dirty crankcase gases out of the engine and burn them. Under normal operating conditions (and except for engines with really bad piston rings or wear) there should almost always be negative pressure under the valve covers ...
 

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D76G12 said:
Things haven't worked that way for a long time! PCV systems are designed to tap into the vacuum from the intake to pull the dirty crankcase gases out of the engine and burn them. Under normal operating conditions (and except for engines with really bad piston rings or wear) there should almost always be negative pressure under the valve covers ...
Yep, D76 is right on that one. The breather is just that, a breather. To allow fresh air back into the engine and back out thru the pcv. If it weren't vented this way, you'd end up with major leaks because crankcase pressures wuold blow out seals and gaskets. There aren't too many sealed systems that I've seen. A majority of cars out there are vented with a pcv system.
 

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rix444 said:
'96, 3.6, I have negative air pressure in the valve covers. At the back of the valve cover on the passenger's side is a breather tube that connects to the large air intake. If I remove the tube and put my thumb over the hole in the valve cover, there is a vacuum. This seems backwards to me. Usually the air under the valve covers is positively pressured due to positive crankcase pressure? What's up?
The tube you speak of is called the "make up air hose" or "vent hose". Positive pressure at this hose is a very bad thing. Oil vapors will get into your throttle body and cause your IAC valve not function properly, just to name one problem.. If you have vacuum at this hose , then be happy.
 

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All are correct. Just to add, Positive Crankcase Ventilation means that all the gases, fumes etc. developed in the crankcase are vented out of the crankcase and burned in the engine.
This is accomplished by pulling fresh air into one side of the engine, through and out the other side via the PVC valve to burn up in th engine. The PVC valve controls the amount of vacuum the engine pulls on the crankcase so that the engine does not die from not having being able to maintain enough vacuum.
 

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dodgerunner said:
The PVC valve controls the amount of vacuum the engine pulls on the crankcase so that the engine does not die from not having being able to maintain enough vacuum.
Sort of. Engine operating conditions affect how much vacuum(ie. idle, WOT, etc..). But what the pcv valve does is regulate the flow of blow by gases based on that vacuum. The pcv valve responds or reacts to how much vacuum is applied to it and the appropriate amount of crankcase vapors flow thru as a result. This is way its important to replace the pcv valve with the correct one. Get one for the wrong engine and you'll end up with a pcv that flows too much or not enough. Neither of which will be good.
 

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Discussion Starter #10
ok, I'm with you on the PCV valve and it's function. But I'm talking 5# - 6 # of vacuum! and it's not constant. If I remove the oil fill cap, there is no vacuum, because fresh air is being drawn in through the 'breather tube'. If I replace the cap and remove the breather tube, there's just a slight draw of air. If I put my finger over the breather tube and plug it, the vacuum slowly increases to the point that it will collapse the tube. Now that I'm writing this, I'm suspecting a problem with the PCV, but it could also be a sign of leaking around the seals of the intake valves. On the other hand, maybe this is how it should operate. How much vacuum is too much? or more specifically...how much 'breathing' should be happening? It takes about 8 seconds to create 5# of vacuum, once the breather tube is plugged. That an enormous quantity of air being sucked out the the crankcase, very quickly. In reverse, if you pumped air into the breather tube, it would take an air compressor 10 seconds to pressurize that engine block to 5psi.
 

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5psi and 5" of vacuum are no where the same. Totally different values.
What you reading it really no that much and a vacuum never builds up in the engine.
You worring about something that does not matter. Change you pvc is you want to do anything.

BTY think of it this way. With your mouth you can suck about 1-2" of vacuum max so 5 is not really that much..
 

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Flow is the important thing with the PCV system, not vacuum. With engine off or during a back fire, the valve is completely closed and there is no flow. At idle or at steady cruising manifold vacuum is high, valve will be partially open with relatively low flow. Under acceleration manifold vacuum is low, and opens the valve all the way. This is the magic of the valve. If you do not see any oil vapors coming out of make up air hose or if you don't see any oil in the intake where that hose attaches, then your PCV system is alive and well. If you constantly drive the car hard you might see oil at that connection even with a good PCV system.
 
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