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Join Date: May 2019
Location: Oklahoma 74432
Posts: 315
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Deceleration Valve Help
I have a 1980 911SC with a question about the deceleration valve. Mine is capped off. Is this correct? According to the Bentley Manual it appears there is a vacuum line running from the valve up to the throttle body. Any and all help is appreciated. Thanks
Last edited by TracyC; 09-07-2019 at 02:04 PM.. Reason: Adding a picture |
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09-07-2019, 01:54 PM
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Join Date: Jul 2012
Location: GA
Posts: 919
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It's an emissions device. A lot of them got capped because people didn't like how they tended to artificially hold revs high when you snapped off the throttle and created a high vacuum. Very common. You could reconnect it and see if you like it or not. As the valve ages it tends to create high idle problems too. I have a "new" old stock one connected to my car and sometimes it sticks open. They are problematic.
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09-07-2019, 02:19 PM
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Location: Oklahoma 74432
Posts: 315
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09-07-2019, 03:09 PM
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Decel valve.........
Quote:
Tracy, A good working decel valve is NC (normally closed). When the engine creates too much vacuum (approx. 22” Hg) during deceleration, the decel valve opens up and reduces the vacuum. If you disable the decel valve by plugging the vacuum line that operates the valve, your motor will lose the ability to release the excess vacuum inside the engine. What most people don’t understand about a decel valve is how it operates. The vacuum line to the nipple of the decel valve is the one that controls the opening of the valve. So you have to check if the decel valve is good or not with or without vacuum. There is a very simple method to test your decel valve if it leaking or not. If it defective or leaking, it could be adjusted to make it seal. Tony |
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09-08-2019, 06:19 PM
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PCA Member since 1988
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I've had other cars that had a decel valve or some kind of throttle damper that prevents the throttle from closing quickly, and it's really annoying to have to pause between gears to wait for the RPMs to drop enough so that you don't get a jerk when releasing the clutch.
The purpose of a DV or throttle damper is to prevent the throttle from closing quickly and manifold pressure from changing quickly, because rapid transitions cause changes in AF ratio and thereby increased emissions. Either a DV or throttle damper eases the transitions by closing the throttle more slowly or allowing air to bypass the closed throttle plate for a short time. Effectively, they do the same thing. Of course, modern computer-controlled cars don't need these band-aids because the AF ratio is tightly controlled in realtime by the computer's maps and oxygen sensor feedback. Therefore, I recently decided to remove my decel valve completely. I have a 1980 SC engine in a 1973.5 911T. The original 2.4 CIS engine didn't have all those emissions controls on them, so I won't have any problems with emissions inspections. Also, when I built the engine, I took off the auxiliary air and other emissions plumbing on the front (front of the car) of the CIS too, so I don't have that crap in the way either. I plugged the nipples that it was connected to on the throttle body and vacuum T, and it runs fine. In fact, the RPMs on my engine dropped quickly before I removed the DV, and I can't tell that taking it off makes any difference, so I suspect it wasn't working as advertised in the first place. Well, one less thing to get in the way or cause problems! As the famous inventor and GM Chief Engineer, "Boss" Kettering frequently said, "the part not added costs nothing, weighs nothing, and won't break." Last edited by PeteKz; 12-15-2019 at 08:09 AM.. |
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12-15-2019, 08:01 AM
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12-15-2019, 08:50 AM
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