vacuum question

[idiocrates]

Okay.....I understand the basics of the crankcase ventilation system on the f-134.....up to a point. And that point is where the pcv valve goes into the "t" fitting between the vacuum pump and the line to the intake manifold. If the vacuum pump is pulling a vacuum....and the port below the carberator is pulling a vacuum what's to keep the pcv valve from staying closed all the time? I realize there is a spring behind the valve but is this all that keeps the valve insert in the pcv in the proper position......and then how important is it that this spring be of the proper strength......and is its strength based on vacuum at both the pump and the intake manifold.....and what happens to this balance if the vacuum pump is not used at all? The reason I'm asking is......one side of my pump.....the side that went to the vacuum wipers was capped off and plugged. The other side was plumbed to the "t" fitting behind the pcv like normal. As I'm reassembling my engine should I continue to use the vacuum pump to "t" fitting connection.......or just scrap the vacuum pump part and plumb my pvc directly to the vacuum port on the intake manifold? Also......if both ports on the vacuum pump are suction......where does the evacuated exhaust go......or is one side suction and the other exhaust? And if the pump is in the system to compensate for low vacuum conditions in the engine then wouldn't the pump be pulling the pcv fumes away from the intake manifold port and sending these vapors to......uh.......where? Well, anyway.......as you can see.....I'm pretty confused.....so any discussion on this vacuum system would be greatly appreciated. Thanks. Oh yeah.....and just fyi.....this jeep has no fording equipment on it at all.......and I will accept any old plain jane "put it back on or else" answers as well as "mine works fine without any of it" answers....I just want to make sure the engine runs right when I finally get to that point.

Jim

[wesk]

Let's start with the definition of vacuum. It is a low pressure which is usually less than the atmospheric pressure so air flow will almost always be towards the vacuum source. In our case the vacuum source is the intak manifold. It just plain sucks! Now if we move upstream to the "T" above the fuel pump you see it sucks the air flow from the PCV valve into the intake manifold. What's in the intake manifold ends up in each cylinder when the intake valves open and it all gets burned and sent out the exhaust pipe.

If you look closely atmoshperic pressure is free to enter the dipstick tube and valve cover from the air cleaner crossover pipe and from the vent on the center of the fuel pump body. This air flow picks up all the fumes and vapors in the crankcase and they are sucked out through the PCV valve to the intake manifold and burned.

Now you are wondering why we have that vacuum pump mounted with the fuel pump. Well we need it to boost the vacuum (suction) from the intake manifold so it will SUCK better on the wipers and the distributor housing vent line. This means improved performance for the wipers which if they only had the SUCK from the intake manifold would not run as well.
This extra SUCK does nothing for the PCV valve which is further UPSTREAM on a seperate tube off the "T".

Now for the inside of that PCV valve. It allows restricted flow towards the intake manifold all the time. This is called a calibrated suction leak. The small piston in the valve has a hole drilled into it that allows the air to flow through it constantly. There's a very light duty spring behind the piston. This spring will allow the piston to move off it's seat and let even more airflow go to the intake manifold when you step on the gas which suddenly increases the amount of SUCK from the intake.
_________________
Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php

[idiocrates]

Thanks for the info, Mr. Knettle. Okay.......so if I'm reading this correctly......other then improving the operation of the vacuum wipers.......the vacuum pump does nothing to improve engine performance.......right? Or is there some improvement in having an operating pcv system (through the vacuum provided by the pump) prior to the establishment of normal intake manifold vacuum?

On the drawing....you've got a red dashed line from the vacuum pump going towards the engine. Does this mean that the "exhaust" of the vacuum pump goes back into the crankcase via the cam operated lever openning? And the two black arrows going from the vacuum pump to the "t" fitting......are these arrows pointing the right way to indicate air flow? Just seems to me like the direction of the arrows for pipes "V" and "X" should be pointing in the same direction...aka.....toward the pump. See to me this looks like pressure......which is also effective in creating vacuum since essentially it would be at a higher atmospheric pressure than the surrounding air and certainly much higher then the weak vacuum of the intake manifold. I guess it just all depends on where you're standing.......high pressure from pump......normal pressure from crankcase.....and low pressure from intake manifold. I agree.....this vacuum stuff sucks.......but that can only mean somewhere it has to be blowing as well!

And...so....if my distributor is not vented......and I don't have vacuum wipers......is there any justifiable reason to replumb the vacuum from the vacuum pump to the pcv system "t" fitting?

When I got this m38a-1 it had a hose from the oil fill tube to the valve cover but on the other side of the fill tube there was just an open hole. Obviously this isn't right inasmuch as dirt and debri could easily enter the engine right down the tube. I fixed this problem at first by just plugging the hole in the fill tube. Now I realize this hole (with its tube to the air intake pipe) is the fresh air supply for the crankcase ventilation system so I have now fixed this by making a connection between the fill tube and the air cleaner pipe. That gets me up to the "t" fitting and all I really have to do here is decide whether or not to make the connection to the vacuum pump or just plug this end of the "t".

Again.....thanks for the help, sir.....and the diagrams......they've helped tremendously.

Jim

[wesk]

[idiocrates]

Alrighty then.....answered, answered......and answered. Got it....and understood. Thank you, sir!

Just to follow the kiss principal I will probably go ahead and replace my combo pump with a simple fuel pump only unt and put the existing one on the shelf as an emergency spare.......along with all the tractor parts that are slightly worn but still servicable.......if I could just figure out which piece of equipment it came off of. I can only imagine the fun my kids are going to have when they try to figure out how the heck that fuel pump goes on a Case tractor! Thanks again, sir for all the help.

Jim