Vacuum limiter (decel valve) delete
04-27-2005, 12:22 AM
#1
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Vacuum limiter (decel valve) delete
I did this before with the '81, but now it's the new car's turn, with pics.
This thing is useless, expensive to replace, and it's removal gives more room to work. (Removing it hasn't shown any change on emissions tests.)
Vacuum limiter, versus vacuum cap, 5/8" and 3/4" rubber heater hose blockoffs (from any parts store).
Removing the plenum makes it easier.
Loosen clamps at throttle body, pull off, remove hoses, then remove through front opening.
Push on caps. Done. (You may have to raise the idle slightly, if the limiter was leaking before.)
This thing is useless, expensive to replace, and it's removal gives more room to work. (Removing it hasn't shown any change on emissions tests.)
Originally Posted by PorKen
The vacuum limiter (or decel valve) is used to burp air past the throttle when decelerating with the throttle closed. It's a hold-over from the K-Jet system.
The idea is to help burn gas which has been injected but unburned because there is not enough air after the throttle is suddenly closed. This unburned gas leads to increased hydrocarbon emissions.
This problem is more pronounced on the manual cars, but for some reason the valve was included on the auto and manual 928's pre-83; 83 and 84 have them only on manuals.
I removed mine. After I removed a bunch of stuff and my vacuum was stronger, this unit cycled at idle, up and down, up and down.
The idea is to help burn gas which has been injected but unburned because there is not enough air after the throttle is suddenly closed. This unburned gas leads to increased hydrocarbon emissions.
This problem is more pronounced on the manual cars, but for some reason the valve was included on the auto and manual 928's pre-83; 83 and 84 have them only on manuals.
I removed mine. After I removed a bunch of stuff and my vacuum was stronger, this unit cycled at idle, up and down, up and down.
Vacuum limiter, versus vacuum cap, 5/8" and 3/4" rubber heater hose blockoffs (from any parts store).
Removing the plenum makes it easier.
Loosen clamps at throttle body, pull off, remove hoses, then remove through front opening.
Push on caps. Done. (You may have to raise the idle slightly, if the limiter was leaking before.)
04-27-2005, 12:47 AM
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Thanks, I think I might do that on my '82 5spd. You said "The problem is more pronounced on the manual cars". Have you done it before on a 5spd (the car in the pictures looks like an auto)?
04-27-2005, 12:53 AM
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That was something I read at the time of the quote. The manual cars drop to idle faster, so the vacuum level can rise faster.
I haven't done this myself on a manual. I think that Porsche may have actually deleted it entirely by '84, on both auto and manual cars.
The valves are usually DOA or tired and oversensitive, anyway.
I haven't done this myself on a manual. I think that Porsche may have actually deleted it entirely by '84, on both auto and manual cars.
The valves are usually DOA or tired and oversensitive, anyway.
04-27-2005, 07:28 PM
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After PorKen's prior suggestion I removed mine on my 1980 5 speed manual with no detrimental effects. I have been driving it this way for about 6 months now. If you drop off the gas hard and put in the clutch it used to have a strange high approach to idle. Now it drops a bit lower than idle but stabilizes quickly.
04-27-2005, 07:59 PM
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I would think so. Look on the passengers side under the spider. It is the obvious flying saucer like thing (see picture above) attached to the intake in two places. I removed it. Saves weight :-)
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04-27-2005, 08:11 PM
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Pull it out and use it as a paperweight. The air intake gets capped as shown, and the small hose can be removed and capped off at the T (or 4 way) splitter. I'll upload a picture in a few minutes.
04-27-2005, 08:22 PM
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Here's a close up. The silver blob on the top is the front passengers side spider tube. You will see I didn't use the caps like PorKen, instead I used smaller caps and just hose clamped them in on the vacuum limiter side. The reason for this was I was able to do it without pulling the spider. I think the whole job took 45 minutes once I had the right sized rubber caps. I had an intermittant surging idle which is now gone. I believe this unit played at least a part in the problem.
04-27-2005, 11:17 PM
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D-Jet, K-Jet, L-Jet, whatever it takes.
On K-Jet especially, the original intention may have also been to keep from pulling extra gas out of the injectors, when you drop the throttle.
Myself, I always try and ease out of the throttle, and only drop it when I want to hear the crackle and pop in my big-ol' cat-less exhaust.
On K-Jet especially, the original intention may have also been to keep from pulling extra gas out of the injectors, when you drop the throttle.
Myself, I always try and ease out of the throttle, and only drop it when I want to hear the crackle and pop in my big-ol' cat-less exhaust.
04-28-2005, 04:56 AM
#13
I took the vaccum limiter valve out of my US 79 5sp and the other valve associated with it the decel valve.
Previous to removal and after fixing all the vacumm leaks the idle surged up and down by about 200 rpm, and when you gave it a good rev the rpms would take for ever to drop and would hang at 1400 rpm for like 20 sec.
Post removal effects were:
- after a good warm up period idle sold at 800 rpm
- the rpms drop quickly after a good rev of the engine but stop dead @ 800 rpm
- way better throttle response
- all around better
Although on warm up the ob is a little grumpy.
Previous to removal and after fixing all the vacumm leaks the idle surged up and down by about 200 rpm, and when you gave it a good rev the rpms would take for ever to drop and would hang at 1400 rpm for like 20 sec.
Post removal effects were:
- after a good warm up period idle sold at 800 rpm
- the rpms drop quickly after a good rev of the engine but stop dead @ 800 rpm
- way better throttle response
- all around better
Although on warm up the ob is a little grumpy.
04-28-2005, 05:22 AM
#14
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It sounds to me that without this widget you might have unburnt fuel passing through the engine into the cat, where it could cause a fire or otherwise damage the cat.