PCV valve & Engine Vacuum
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Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
PCV valve & Engine Vacuum
I have a strange problem. My PCV valve causes my engine vacuum to drop. I tried
two different valves. The small one causes my vacuum to drop to 5 in-Hg, while
the larger one caused my vacuum to drop to zero. My normal manifold vacuum is 11 inches-Hg.
If I take the PCV valve out while the motor is running, there is no pressure in
the valve covers. It seems like the valve cover area is open to atmosphere.
Should my breather vent be restricted? Or do I have too many leaks in my valley cover?
two different valves. The small one causes my vacuum to drop to 5 in-Hg, while
the larger one caused my vacuum to drop to zero. My normal manifold vacuum is 11 inches-Hg.
If I take the PCV valve out while the motor is running, there is no pressure in
the valve covers. It seems like the valve cover area is open to atmosphere.
Should my breather vent be restricted? Or do I have too many leaks in my valley cover?
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Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
I have been reading up on PCV valve operation. The valve will open at low vacuum levels. My 11 in-Hg vacuum is not enough to close the valve. I need to reduce the throttle plate opening at idle to get the idle vacuum higher, or get rid of the PCV valve and run a draft tube.
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Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
The low manifold vacuum does affect my power brakes. At 11" it's OK, but at 5" I need a lot more foot power to stop.
I can't get any more vacuum by adjusting the TB. I think that I need a draft tube.
Lead Driver
Joined: Nov 2000
Posts: 6,628
Likes: 345
From: Albuquerque
I had a lot of trouble getting a good idle on my setup. What helped the most was to create a 1/8" restriction in the intake side of the PCV system. Using a vacuum gauge hooked on the dipstick tube I would look for about 5" of vacuum.
This can also help with ring sealing.
This can also help with ring sealing.
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Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
According to Comp Cams 11" of vacuum is normal for my cam. It's the PCV valve that's the problem. The valve will not close with that low vacuum. I went to the auto store and purchased a PCV valve for a high performance 427 Vette. This was supposed to fix my problem, but it didn't. I also too the PCV valve from my '83 F100 with a 302 carb motor. The vacuum went to Zero with that unit.
Now I am using a Road Draft Tube, and I get 11" of vacuum. My idle has been good all the time. With Mass Air EFI, low manifold vacuum is not a problem. It relies on the flow through the MAF, not vacuum like a carb.
Now I am using a Road Draft Tube, and I get 11" of vacuum. My idle has been good all the time. With Mass Air EFI, low manifold vacuum is not a problem. It relies on the flow through the MAF, not vacuum like a carb.
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