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Decel valve question

My decel popping/crackling seems to be a bit too much lately, and in searching I came across some references to the decel valve. I thought I would check mine with a miti-vac gauge to determine that it was functioning.

When I attempt to build vacuum in the diaphragm in the photo, I cannot. Not even a little bit. Is that indicative of a failed valve, or is the thing normally open somehow and I am not testing it properly? Finally, assuming it is bad, would replacing it likely reduce some of the popping down to what I seem to remember as "normal" levels?

On Edit- wow, this new camera's out-of-the-box resolution/image size is HUGE. Will need to look into that.

http://p6.hostingprod.com/@yorktown6.com/porsche/DSC_0130.JPG

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Old 08-14-2010, 08:34 AM
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It should hold vacuum and not leak down, and it should open the connection to the 2 larger hose connections on the other side at around 21 inches of intake manifold vacuum which it will only see when decelerating with the throttle closed in a low compression 930.

Replacing it would be the easiest way to lower the popping back to what it was.
A new one is unbelievably expensive if you can find one but you should be able to find a used one cheaply with a wanted ad..

You should replace all those old dry rotted origonal fabric covered vacuum lines. The rubber hose underneath the fabric wrap cracks and then leaks and you can't see the dried rubber cracks with the fabric over it.
You can see it's the origonal weathered vacuum hose on the left vacuum solenoid that switches the left and right side air pump diverter valves if they are still there.
Old 08-14-2010, 10:46 AM
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Thanks Jim. Digging around I see several different part numbers for different year CIS cars for the vacuum limiter. A pelican IM'd me on one they had for a non-turbo car and I figure on trying it. I'm assuming they open at slightly different vacuum levels... but that functionally they perform the same task. Any ideas on the differences?
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Old 08-15-2010, 03:31 PM
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The decel valves I have seen for non-turbo CIS cars have two hoses that attach to the back side and one on the front. The turbo has a single hose fitting on both sides.
Old 08-15-2010, 04:22 PM
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No, I'd guess the same as you. Normally aspirated cars with higher compression have higher intake manifold vacuum at idle than a 930 so the decel valve opening vacuum would have to be higher for those cars.
A 930 decel valve would probably open at idle in a moderate to high compression SC. Then it would stay open and idle real high.

There was one on the for sale page the other day when you posted about this for $40 or something like that. I think it was from a 930.

If you wanted to experiment, search ebay for "blow off valve"
There wil be hundreds of them and some of the small adjustable ones with 19mm hose fittings might work.
Old 08-15-2010, 04:28 PM
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Quote:
Originally Posted by OSI930 View Post
The decel valves I have seen for non-turbo CIS cars have two hoses that attach to the back side and one on the front. The turbo has a single hose fitting on both sides.
The 930 deceleration/vacuum limiter valve has 2 hoses on the back and the vacuum line that moves the diaphram and opens the valve is on the front side.
Old 08-15-2010, 04:32 PM
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Holy crap, I almost lost my cookies navigating through the huge picture in your original post. Great resolution, though,

I think your questions have been answered. I did notice, however, that your left hand IC mount shows a crack. Mine did the same, some previous owner helicoiled it, but still it eventually broke clean off....uh-oh. I drilled it, tapped it, applied JB weld and screwed in place a long enough stud to mount the IC to. Just something to tuck away until the day comes that you end up needing to repair it
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Old 08-15-2010, 06:12 PM
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Quote:
Originally Posted by JFairman View Post
A new one is unbelievably expensive if you can find one but you should be able to find a used one cheaply with a wanted ad..
$500, special order from the Fatherland....
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Old 08-15-2010, 06:27 PM
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Thanks JFairman. I am not sure why I thought that there was only one hose the back as I just had everything off to do the dump valve seals. It makes sense that the valves are different between the turbos and other CIS cars based on manifold vacuum and compression.
Old 08-15-2010, 07:39 PM
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I removed my valve when I installed my kokeln ic, with the updated blow off valve. I was considering reinstalling it. However, while testing it I could not get the valve to open with vacuum. So I tale it some folks have just used another bov say going from a tee off between ic and aar, and the original nipple on the manifold. So get one that opens around 21 in. Should be good to go, huh?
Old 08-15-2010, 08:37 PM
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Ok, just for giggles I reinstalled the decel valve. I got it to start working again by spraying a little carb cleaner in the vac nipple. Hopefully that won't gum it up worse, anyway took it for a spin and got a lot less popping, but still enough to keep up the 930 image! Everyone seems to hate on the decel valve, but I kind of like it seems like the shifting is a little smoother. I will keep it on for a while. Can always change it back.
Old 08-16-2010, 01:25 PM
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