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Discussion Starter #1
Hello,

Have a 1969 and I am chasing a vacuum issue. I assume I have a leak because I am only developing 6psi at idle and if I turn off the car before the headlights, the headlight doors will not close. Anyway, I am trying to chase the leak and I was pinching off hoses trying to isolate a change in the idle. A friend was helping and pinched the PCV hose and the change in the idle was dramatic. He thought my PCV valve was bad, but I disagree.

If you came up behind me and choked me, wouldn't I have a hard time breathing? Isn't that the same philosophy with a PCV and wouldn't you expect the idle to stumble if you pinch the hose?

I am not sure what all a PCV valve does, but venting the block is what I believe the primary function, so if you choke it, it would affect the idle, right?

Thanks!
 

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The Positive Crankcase Ventilation Valve ,.....Positively Ventilates the Crankcase.....via a valve. The valve is spring loaded, at idle only a slight vacumn is pulled on the crankcase,.....under load, higher rpm the spring allows the valve to open more because there is less vacumn with the pedal down...allowing more breathing when you need it.

So when you tune your carb the PCV vacumn “Leak” is calibrated in., but it is always some leak.....yes squeezing off that small leak at idle may be noticeable...but a dramatic change may indicate that the valve is stuck open from gunk......if you then squeeze the hose the change could be much bigger....so it is hard to say from here how your change was, but if you substitute a known good valve or buy one for $5. You can easily check and see.

If clamped off and the change is dramatic enough, it may bear changing....

Correct is a slight leak of vac at idle thru the valve....just make sure you keep a PCVQ your engine must have ventilation
 

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Discussion Starter #3
Thanks, so if my PCV is open too much at idle, would that contribute to such a low vacuum at idle? I was not looking at the tach, but when I clamped the hose on the PCV it had to drop at least 150 rpm. That seems excessive compared to what you stated. This is a brand new PCV in a brand new engine (just about 1k miles). Now we all know 'new' PCV does not mean good PCV.

My brakes are like manual brakes at idle, would a PVC stuck open cause such a rapid bleed down of stored vacuum? Seems unlikely, and too much to hope for that a bad PCV would be causing my vacuum issues. It would be a cheap and easy enough thing to test.

Thanks again,
Mike
 

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A PCV valve that is stuck open or has a weak internal spring would cause a large vacumn leak at idle. Also the wrong PCV valve will not get it right. As if your motor is bored and stroked etc, the valve was for the stock engine.

I agree to check the brake booster and brake booster check valve, ez checks. And if the PCV valve off brake booster check valve they are but a few dollars. Start checking and verifying the ez stuff, and then move to the next.

I recommend the ME Wagner dual flow PCV valve, that is what I use, but that is another discussion. First you have to get your vac up beyond six hg, there is a big leak somewhere, and based on what your cam specs and build is what Vac can you expect, approximately.

Figure that, then see if blocking the lines gets you close.

You will get it stay with it!
 

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Discussion Starter #6
Thanks Lemans Guy. Sorry I didn't see this earlier. Curious, how do you use the ME Wagner valve? Did you relocate the PCV to the valve cover? Mine is in the valley pan and I don't think I can get one in there unless is has the hose coming out the top like a stock one. I am at 461 ci and I have a cam with specs a touch hotter than a RA III but not as aggressive as a RA IV.

I will start checking other stuff, just rebuilt engine last year and changed brake booster and check valve (but I know, new don't mean good). My back was screwed up and I was not able to trouble shoot it and it seems there has always been anotehr problem to deal with. Car was running hot and now I just realized that when I fill my tank, it leaks. Either I have a hole in the tank or the sending unit gasket is bad.

Frustrating and trying to get the car ready for a big show in 5 weeks.

Thanks,
Mike
 

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A big vacuum leak at idle will certainly cause your symptoms, and if pinching off the hose "corrected it", made it run better, and raised the gauge reading then that's pretty conclusive evidence of where the problem is.

Bear
 
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