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Old 02-10-2011, 10:19 PM
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Vacuum leak or...?

I seem to have a major vacuum leak that I cant figure out. I have a 400 c.i. with an Edelbrock 1406 4 bbl carb. When you are sitting at idle you can hear what sounds like a vacuum leak, but for the life of me I cant seem to track down where it is coming from. I noticed that when I crimp the hose going from pcv valve to the carb it almost goes away. If I unhook the hose from the pcv valve altogether it runs MUCH better. The pcv valve is brand new and the carb is freshly re-built. Anybody had a similar problem? Is there something obvious that I am not seeing? I am probably going to (temporarily) drive it like this just because this is my only running vehicle. I know its probably not a good idea to run around with that hose unhooked but it runs pretty good that way. If you just plug the hose off (with it still hooked to the carb it goes back to running horrible. Any advice would be greatly appreciated, thanks.




My truck: 79 Ranger F-250 4x4
400 c.i.
4 speed manual transmission
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Old 02-11-2011, 12:08 AM
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The easiest way to spot a vacuum leak is to lean the motor out at idle so that it barely runs, then use a can of carburetor cleaner and spray at suspect areas. If the idle jumps, then you've found a leak.

The PCV hose/poor running issue sounds like blowby. If disconnecting the PCV hose makes the engine run better, then the rings are probably letting more blowby into the crankcase than the system can cycle through. Are you running a breather filter in the other valve cover? It's not obvious why blocking off the PCV vacuum source makes it run worse; perhaps there is some over-rich issue that the forced vacuum leak helps mitigate.
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Old 02-11-2011, 03:55 AM
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The PCV valve IS a vacuum leak, just a controlled usefull one, the hissing noise is just it working. Sounds to me like it just needs to be tuned to run right. The blowby and flow/hissing issues need to be answered with one question, is there a vent/breather in the valve covers?
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Old 02-11-2011, 08:45 AM
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The PCV valve is a controlled vacuum leak, but you shouldn't be able to hear it hiss. If you hear it hiss, that means it's open to the atmosphere (and it is, but not directly). The PCV valve pulls fumes in through the crankcase, and the fumes are displaced by fresh air throuh the breater filter. So while the PCV valve does have a path to open atmosphere, the path is long enough that you shouldn't hear it hiss. If you can hear a hiss from the cab, then that's a sign that there's a direct path between the atmosphere and negative pressure in the intake stream.
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Old 02-11-2011, 10:12 AM
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I guess I don't know exactly what I should try to do next. I suppose I could go buy a can of carb cleaner and spray it all over under the hood and see if there are any vacuum leaks and/or hook up my vacuum gauge and see what it says. So, if I have excessive blow-by, what does that mean? My rings are going bad? What will happen if I just run it with pcv hose un-hooked? I realize it makes a major vacuum leak, but it seems to run good that way. Well damnit...its back to not starting or running at all!! I am at a total loss with this thing.
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Old 02-11-2011, 02:17 PM
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Does it matter which valve cover the pcv valve and breather is on? I didn't think it made a difference...does it? Will lack of vacuum or vacuum leak prevent the truck from starting at all?
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Old 02-11-2011, 05:28 PM
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Does it matter which valve cover the pcv valve and breather is on? I didn't think it made a difference...does it? Will lack of vacuum or vacuum leak prevent the truck from starting at all?
YES YES YES and ohh yeah YES

If you have a breather it must at least be in the opposite valve cover as the PCV, I suggest not having a breather at all with a PCV.

You need to take some time and think about what the PCV (Positive Crankcase Ventalation) does. It connects the vacuum of the intake manifold to the valve cover, which in turn connects to the crankcase, and the other valve cover. This is done so that the blowby gasses that all engines have are sucked into the intake and burned in the cylinder, this is a good thing. It also helps seal the engine from oil leaks and the rings.

So think thru these 2 questions; 1) What happens if the breather is in the same valve cover as the PCV? 2) What happens when there is no breather, just a PCV?

As for it effecting how it runs, well where does the engine get it's air and how does the vacuum leak of the PCV effect that? This isn't terribly complicated stuff. If the air source of the PCV is gone there is less are going into the engine, this has the same effect as closing the throttle.

My suggestion, hook up the PCV, plug the breathers, and go at the carb with a screwdriver and tune it.
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Old 02-11-2011, 06:41 PM
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the breather should be on the opposite end of the opposite valve cover and SHOULD NOT be plugged this will seriously effect crancase breathing . think of plugging the breather as putting a bag over ones head, makes it kind of hard to breath in fresh air
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Old 02-11-2011, 06:54 PM
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I suggest not having a breather at all with a PCV.
This is BAD.

The breather filter is absolutely necessary. When blowby fumes are pulled into the intake stream through the PCV valve, they must be displaced with something. This is the breather filter's job. Blowby is pulled into the intake stream through the PCV valve, and fresh air takes its place. It's a cycle, hence the term positive crankcase ventilation.

If you run the PCV valve with the breather plugged and no way for fresh air to be drawn in, the vacuum signal will find another way to equalize the pressure in the crankcase and loosen the seals on the engine, because it's going to try and pull in fresh air any way it can. If you suck all the air out of a balloon, the balloon won't stay the same size. If what you're saying worked, there'd be no way to drink out of a straw

Everything that 79 f350ts has said is correct.
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Old 02-11-2011, 07:12 PM
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Originally Posted by 79 f350ts View Post
the breather should be on the opposite end of the opposite valve cover and SHOULD NOT be plugged this will seriously effect crancase breathing . think of plugging the breather as putting a bag over ones head, makes it kind of hard to breath in fresh air
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This is BAD.

The breather filter is absolutely necessary. When blowby fumes are pulled into the intake stream through the PCV valve, they must be displaced with something. This is the breather filter's job. Blowby is pulled into the intake stream through the PCV valve, and fresh air takes its place. It's a cycle, hence the term positive crankcase ventilation.

If you run the PCV valve with the breather plugged and no way for fresh air to be drawn in, the vacuum signal will find another way to equalize the pressure in the crankcase and loosen the seals on the engine, because it's going to try and pull in fresh air any way it can.

Everything that 79 f350ts has said is correct.
I of course disagree, and will state my case, how much you 2 want to agrue and hijack this thread is up to you.

The big incorrect ussumption both of you are making is that bringing fresh air into the crankcase is a good thing, it is not. Both of the reasons you stated are actaully reasons to not have a breather.

First the idea of freash air being a good thing, what does freash air have in it that blowby gasses don't, water vapour and oxygen. Both of these are bad for the internals of the engine and the oil, The inert gasses of blowby are better.

Second, a vacuum in the crankcase is a good thing, we should call PCV PCVacuum not ventalation. Vacuum in the crankcase is benificail for the same reason racers use vacuum pumps and dry sump systems. Maintaining a vacuum helps seal the engine and reduce windage drag. For us the sealing aspects are the big benifit. Many engines seals aren't all that great, most of all the crankshaft seals, these will always leak a little, better they leak in then out, pulling in air then leaking oil. Secondly by maintaining a vacuum under the pistons the rings are always being pulled in the same direction reducing flutter, blowby, and oil consumption.

I quarentee if u have a small oil leak or consume oil and have a PCV with a breather, plugging off the breather will reduce or eleminate it.
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Old 02-11-2011, 07:28 PM
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first off nobody is "hijacking this thread" simply answering the op's question. second were talking about a street motor, not a race engine, big difference . third you can say we "should" call it PCVacuum all day long, but it's called PCventalation for a reason. if you disagree fine thats your right but no need for rudeness and your not helping the op, which is why were supposed to be here in the first place
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Old 02-11-2011, 07:42 PM
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first off nobody is "hijacking this thread" simply answering the op's question. second were talking about a street motor, not a race engine, big difference . third you can say we "should" call it PCVacuum all day long, but it's called PCventalation for a reason. if you disagree fine thats your right but no need for rudeness and your not helping the op, which is why were supposed to be here in the first place
1) It would be a hijack if it just became a vent or no vent arguement, sure it's only a little off topic but not really his concern or input.

2) Little known fact, street engines and race engines work with the same basic technology, what works great in one often does the same in the other for the same reasons. They all go suck, squish, bang, blow.

3) Not being rude, if you can't make a logical argument don't dismiss it by calling me rude.

4) Suggesting a better way absolutely can help the OP, it would solve all his problems. Incorrect superstitous advice won't, doing the same wrong things over and over again helps nobody.
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Old 02-11-2011, 07:52 PM
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there is nothing illogical , wrong, or superstitious about the advice from myself or FMC400. you dissagree , fine , the op can draw his own conclusions from what is posted. not going to be drawn into a p$@@&*g contest, as previously posted thats not what this forum is for, THAT would be getting off topic
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Old 02-11-2011, 08:38 PM
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mr-ranger 79 , i'm afraid arguing back and forth about pcv function has gotten everybody away from your original problem. sorry bout that. now whether or not a breather is deemed necessary or not , we all should agree the presence of one would not cause your problems . they all came factory with no issues. the engine gets its air through the carb, which is more than likely the culprit. by running better with the hose off versus plugged your air/fuel ratio is probably off{too rich or airflow choked} check adjustment of mixture screws and if equiped check choke to make sure it opens fully, the additional air thru the open pcv hose is merely making up for the lack of airflow thru the carb. hope this helps, and sorry we all got off track
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Old 02-11-2011, 08:52 PM
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LOL...No problem. I think I learned plenty through all that. The thing that really confuses me though is that it would not start this morning no matter what I did. Then I loosened the dizzy and rotated it (quite a bit) clockwise then it fired right up and ran pretty good. It was timed at 12 degrees, who knows what it is at now, but I need to check it. For some reason it does seem to enjoy the pcv valve being unhooked. I guess I will play with it some more tomorrow with better luck I hope. Another question that I have that is off topic a bit is under the passenger side of the truck there are two canisters. I figured they were for the fuel tank breather hoses. They are hooked up (I assume) to the fuel tanks. But when the lines come up to under the hood they just up and end and arent hooked to anything. I was looking at the extra engine that originally came out of this truck and I noticed that it had an air pump. Is that what those canisters plugged into? Are they even needed? They cant possibly be working right now considering they dont go anywhere. Was just curious.
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Old 02-11-2011, 08:52 PM
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