breather cap: why?
- Thread starter falcon fanatic
- Start date
Yeah, I have the aluminum valve cover, and good amount of oil comes out my PVC, thus prompting this posting. I think I'll install a separator of some kind between the PCV and the intake manifold.
That thing about the breather being on one bank of a v-8 and the PVC on the other makes more sense. On my car, of course, they are just at opposite ends of the valve cover, hence my doubt about the effectiveness of the system.
Finally, where is the road draft tube located on engines that have one?
That thing about the breather being on one bank of a v-8 and the PVC on the other makes more sense. On my car, of course, they are just at opposite ends of the valve cover, hence my doubt about the effectiveness of the system.
Finally, where is the road draft tube located on engines that have one?
60s Refugee
Well-known member
I didn't think about JB weld. I suppose it could hold an improvised baffle plate. Good idea!
Harry
Harry
On the block forward of the distributor. There should be a freeze plug on your engine.
They do spew oil and smoke out when in operation. I have one, and it is pretty aggravating. When the engine is hot, you can smell and see smoke coming from under the hood. I thought that the car was on fire once.
Plus there is sometimes a line of soot underneath the car. I'm going to put a PCV on the motor when I finish with the really important stuff.
They do spew oil and smoke out when in operation. I have one, and it is pretty aggravating. When the engine is hot, you can smell and see smoke coming from under the hood. I thought that the car was on fire once.
Plus there is sometimes a line of soot underneath the car. I'm going to put a PCV on the motor when I finish with the really important stuff.
66 Fastback
Famous Member
PCV is not needed as much on modern vehicles. The clearances are tighter and there are less unburned fumes and blowby in the crankcase, so there is less load for it to ventilate. The potential for an explosion back from the carb necessitated the check valve. Fuel injected cars do not have that risk, so they eliminated the check valve and in many cases went to just a sized orifice.
My '61 Vette was originally equipped with a vented oil filler cap and a road draft tube. The California cars were equipped with a PCV system. At high rpm's (6500 rpm redline) the solid lifter cars had problems with oil spewing out the breather cap. GM issued a tech bulletin to install a non-vented cap on the solid lifter engines. So it make you wonder how necessary the vented cap is. As an auto-engineer pointed out, on a Corvette discussion board, this "addressed" the oil mist problem and made the customer happy, but it did not facilitate the operation of the PCV system.
Here is a portion of his post regarding the necessity of fresh air venting.
Keep in mind that modern motor oils have detergents and anti-sludging additive packages that address some of the problems that the early PCV systems helped solve.
Doug
My '61 Vette was originally equipped with a vented oil filler cap and a road draft tube. The California cars were equipped with a PCV system. At high rpm's (6500 rpm redline) the solid lifter cars had problems with oil spewing out the breather cap. GM issued a tech bulletin to install a non-vented cap on the solid lifter engines. So it make you wonder how necessary the vented cap is. As an auto-engineer pointed out, on a Corvette discussion board, this "addressed" the oil mist problem and made the customer happy, but it did not facilitate the operation of the PCV system.
Here is a portion of his post regarding the necessity of fresh air venting.
Keep in mind that modern motor oils have detergents and anti-sludging additive packages that address some of the problems that the early PCV systems helped solve.
Doug
your RDT is just in front of the distributor, on the side of the block. Some have only a tube, others (like mine) had a small canister. I removed this and installed a cranckcase filter. This filter allows the engine to suck in fresh air as it now exhaust the hot dirty air through the PCV and into the intake.
There are other threads on this very thing. Years ago when I was trying to figure out how to set this system up correctly, I read about them. Do a search.
There are other threads on this very thing. Years ago when I was trying to figure out how to set this system up correctly, I read about them. Do a search.
This is an interesting discussion and just thought I would throw in my $.02. My knoledge of engines is admittadly more empirical than the result of any formal training. With that said, my understanding of the Positive Crankcase Ventilation (PCV) system has always been that since you have a pump in your engine, constantly working to circulate oil, and since the oil does not fill the entirety of the system (all the area contained withinin the oil pan, valve cover, and oil circulation channels, then you naturally develop oil pressure as well as air pressure in the engine lubrication system.
If the system were closed (no ventilation) then science says the pump would effectively be working to pressurize the two components (air and liquid)into one. Also, if the system were completely closed the pressurized air (more so than liquidl) containing some amounts of oil would look for a path of least resistance to escape and of course carry liquid along the way. A closed system would further inhibit the efficient movement of liquid (oil) through the system for lubrication purposes. Pressurized air would impede the free flow of liquid in the system. Therefore the PCV system is absolutely necessary even today.
I've seen a modern engine (less than 10 years old) that was blowing smoke out the tailpipe, and weeping from the valve covers like crazy. PCV valve was corroded shut, changed the valve both problems went away. Although don't believe the rings and gaskets were better for the wear. I have also seen a crankcase circulation and vacum system so tight, that when you took off the oil fill cap (or even pulled the dipstick tube) while the car was running it immediately died (vacum leak). See BMW and efficiency. To my knowledge the main reason manifold vacum is utilized to evacuate the system is primarily it is efficient and accessible (easy and right there), and second it gives the engine the opportunity to burn a certain amount of the contaminated air rather than venting it right out from under the hood. IMO
Robert
If the system were closed (no ventilation) then science says the pump would effectively be working to pressurize the two components (air and liquid)into one. Also, if the system were completely closed the pressurized air (more so than liquidl) containing some amounts of oil would look for a path of least resistance to escape and of course carry liquid along the way. A closed system would further inhibit the efficient movement of liquid (oil) through the system for lubrication purposes. Pressurized air would impede the free flow of liquid in the system. Therefore the PCV system is absolutely necessary even today.
I've seen a modern engine (less than 10 years old) that was blowing smoke out the tailpipe, and weeping from the valve covers like crazy. PCV valve was corroded shut, changed the valve both problems went away. Although don't believe the rings and gaskets were better for the wear. I have also seen a crankcase circulation and vacum system so tight, that when you took off the oil fill cap (or even pulled the dipstick tube) while the car was running it immediately died (vacum leak). See BMW and efficiency. To my knowledge the main reason manifold vacum is utilized to evacuate the system is primarily it is efficient and accessible (easy and right there), and second it gives the engine the opportunity to burn a certain amount of the contaminated air rather than venting it right out from under the hood. IMO
Robert
Ian,
Yeah, my science can be as shadtree as my mechanic work (I'm an economics <dirty word here lately, guy). That makes sense with it being called a 'crankcase' ventilation system, that combustion or blow by gases as they've been called are the major source of pressure on the system.
Gotta love the beemers, models of efficiency (<economics bias again).
Robert
Yeah, my science can be as shadtree as my mechanic work (I'm an economics <dirty word here lately, guy). That makes sense with it being called a 'crankcase' ventilation system, that combustion or blow by gases as they've been called are the major source of pressure on the system.
Gotta love the beemers, models of efficiency (<economics bias again).
Robert
My honda dies when you pull the oil cap, also. The valve cover is vented back to the intake with no apparent check valve. That serves the PCV function, I suppose. there is no other vent that I am aware of.
Yeah, I make no claim to have in depth knowledge of the PCV system on Hondas, BMWs, or anything other than the ford six at this time 
There is always more than one way to skin a cat, you just need to make sure you understand why completely so you don't leave some key part of that system out.
There is always more than one way to skin a cat, you just need to make sure you understand why completely so you don't leave some key part of that system out.
My 'guess' would be that removing the oil filler cap or dipstick would kill the car depending on where the pcv system obtains its fresh air inlet. On newer cars I think this would be either above or below(within) the carburetor/throttle body. My '01 gas guzzling Dadge pulls fresh air from the air filter element (above the throttle body not affecting vacum system). Taking the oil filter cap off (or forgetting to put it on 
) does not kill the truck. Not much different from my '66, i6 whose element is integrated into the brether cap.
Robert
) does not kill the truck. Not much different from my '66, i6 whose element is integrated into the brether cap.Robert
66 Fastback
Famous Member
The oil pump pressure has nothing to do with crankcase pressure. The oil pumps are positive displacement pumps operating at say 60 psi. There is no where near that kind of pressure in the crankcase that would affect the pump rate. The pump pressure is a function of the pump pressure regulator and the internal bearing & passage clearances.
The old cars had more blow by due to looser engine machining tolerances. So you had more combustion gases and entering the crankcase. The crank also whipped up the oil and vapors. In addition, even though some pistons are going up and some are going down at the same time, you still have some slight surges in "pressure" as the pisotns are going up and down, moving around large volumes of air.
My guess is that the BMW and other cars that die if the oil cap is removed have a closed ventilation system, with no fresh air introduction. As I posted above, the trend on more modern cars was to go away from fresh air as it was not necessary. There is less blow by on modern vehicles and more complete combustion of the fuel than the old carbureted cars of the 60's, so they do not need to vent as much. With the cap removed, oxygen is allowed into the crankcase in an otherwise oxygen deficient environment, so it acts as a vacuum leak and kills the car.
Doug
The old cars had more blow by due to looser engine machining tolerances. So you had more combustion gases and entering the crankcase. The crank also whipped up the oil and vapors. In addition, even though some pistons are going up and some are going down at the same time, you still have some slight surges in "pressure" as the pisotns are going up and down, moving around large volumes of air.
My guess is that the BMW and other cars that die if the oil cap is removed have a closed ventilation system, with no fresh air introduction. As I posted above, the trend on more modern cars was to go away from fresh air as it was not necessary. There is less blow by on modern vehicles and more complete combustion of the fuel than the old carbureted cars of the 60's, so they do not need to vent as much. With the cap removed, oxygen is allowed into the crankcase in an otherwise oxygen deficient environment, so it acts as a vacuum leak and kills the car.
Doug
I don't think that is correct. A inoperative pcv valve will cause oil pressure readings above/below normal. Stuck closed will result in higher than normal oil pressure readings, and worn or always open pcv will result in lower than normal readings. They have to be related.
Okay...that does make sense...my bad
So I guess it's no so much the valve sticking that raises the oil pressure as it is the blow by vapors contaminating the oil causing sludging that can then in turn cause obstruction in oil passages and relief valves raising the pressure?
Okay :idea: , now I believe I get it. Sorry 66 Fastback, wasn't meaning to take issue with you personally, just trying to understand what I've seen happen. Please disregard my poor explanation above...guess I should put more ??? after my thoughts 
Robert
Robert
66 Fastback
Famous Member
The oil pump does not exert any pressure on the crankcase. The suction of the oil pump is the oil pan sump. Ultimately, the oil is discharged / drains back into the sump. There is no net change in the pressure of the oil when it reaches the oil pan at the end of its journey. All pressure is consumed as friction / head loass through the backpressure valve and tight clearances of the bearings. When it squirts out of those passages, it losses any pressure it has and drains back to the sump. After start up and stabilization, the oil level does not change. For the oil pump to increase the pressure, there would have to be an increase in the oil volume in the crankcase. But the oil that is pumped out of the case, returns to the case, so the fluid level does not change.
You are confusing the pressure in the crankcase vapor with the oil pressure. The oil pump is exposed to that same pressure on the suction and the discharge side of the pump. So even if the crankcase had 1000 psi of pressure in it, the pump is still only going to pump its 60 to 100 psig of differential pressure that it normally pumps because that is all the pressure required to move the oil through the engine.
Doug
You are confusing the pressure in the crankcase vapor with the oil pressure. The oil pump is exposed to that same pressure on the suction and the discharge side of the pump. So even if the crankcase had 1000 psi of pressure in it, the pump is still only going to pump its 60 to 100 psig of differential pressure that it normally pumps because that is all the pressure required to move the oil through the engine.
Doug
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