SC decel valve woes

[schoward]

My 81 sc CIS is in good working order, with O2 connected and all components doing their job. The only exception was the decel valve which had the typical issue of causing runaway idle to about 2k or causing the revs to often hang up and stick at around 2k when coming off throttle. I tried at least three valves and also did the vice squeeze on nearly all of them at varying degrees to try and correct.

I finally broke down and bought a brand new decel valve which arrived in genuine porsche box and is the the exact same Bosch part. Brand new out of box - and while the runaway idle when hot issue is fixed, when hot, it occasionally hangs up the revs at 2k coming off throttle or cruising at revs above 2k and won't release back to idle unless you load the engine or blip the throttle a few times. I haven't done the vice crush on the brand new one as it never seemed to help with this particular issue with my other valves.

I know everyone will say plug it, but any other thoughts on how to fix? Decel valve has maybe 200 miles on it. Am I to believe this is what these things did straight from the factory? Does the vice crush fix the 2k revs hang up issue and I need to apply that fix to this brand new part?

I may plug it, but am not giving up yet. I like what it does.

Thanks.

[reesestewww]

Following...

[jason2guy]

its most likely not the decel valve. check the accel linkage, throttle body mechanism, distributor advance, vac advance. did you replace the one with 2 hoses or 3 hoses? the one with 2 hoses can cause the same headache. i have an 81 as well and recently it started misbehaving sorta the same way yours did. i gutted the whole aux air system minus the electric air regulator and just plumbed that part in with some breather hose. now the car starts in half a second and doesnt have a runaway hot idle

[porsche930dude]

Plug it for a while to see if the problem persists. Then youll know if thats truely the culprit

[sm70911]

I had the same problem when a new Decel valve was put in during my recent top end re build. Mark from Exotech told me it would be like that until it settles in. He advised me not to plug the DV. I had a few months of high revs it would sometimes idle nicely sometimes jump up to 2k when cold. I would try to control it, and blipping the accelerator would bring it down. I don't drive the car a lot, except from May until October and it sits covered through the three to four months of winter. Now the car starts up and idles perfectly, and I've never had that problem again. I'm sure if I had driven it more it would have gone away quicker

[tirwin]

Well it is sorta doing what it's supposed to. I think you have 3 choices: (1) live with it, (2) see if it gets better after a break in period as sm70911 suggested or (3) plug it.

It is a fairly simple device. As much reading as I have done on CIS I've never come across anything that suggests it can be adjusted. Of course, I learn something new every day too.

[schoward]

Thanks for the thoughts. I have tried plugging the line and the problem disappears (along with the decel valve function), so it has to be the decel or a vacuum condition that makes the decel act that way. It only does this occasionally when hot so it is a minor annoyance but after $200 for the brand new part, it's a little maddening. I will try and follow the advice above and stay the course for awhile.

On the plus side though, after trying so many of these things (I think 4), I can now swap decel valves in and out in about 15 minutes flat.

[Bob Kontak]

Originally Posted by tirwin

It is a fairly simple device. As much reading as I have done on CIS I've never come across anything that suggests it can be adjusted. Of course, I learn something new every day too.

It can be squished a little with a big honkin set of channel locks. The newer style off on the passenger side rather than the earlier SC "old" space ship looking decel valves.

I cannot see this being the issue though, given new. Lemmie go check out the marks on mine left by the P wrench in 1997 and I will revert with pic.

[Bob Kontak]

Originally Posted by porsche930dude

Plug it for a while to see if the problem persists. Then youll know if thats truely the culprit

Yep. Plug the hose not the valve.

Doubt it is the issue but well worth the try. Just a nail in the little hose.

[Bill Douglas]

Originally Posted by schoward

or a vacuum condition that makes the decel act that way.

I occassionally get this sort of thing happening. I cut about 10mm off the end of the vacuum hoses and reseat them. You can feel that they are a more snug fit. It's a free and quick thing to try

[boyt911sc]

Test your vacuum line to the decel valve by using a vacuum gauge. Or your decel valve might not be sealing well.

Tony

[schoward]

Originally Posted by boyt911sc

Test your vacuum line to the decel valve by using a vacuum gauge. Or your decel valve might not be sealing well.

Tony

Tony

That seems pretty easy to check vacuum level ('T' in a vacuum gauge to the line) . What am I looking for - super high amounts of vacuum - what would be too high? Odd vacuum signature? What could cause too high/strong a vacuum at some points and not others (only does this when car hot).

Also as a doubtful possibility - I have a slightly smaller diameter vacuum hose than typical cloth metric ones on car (FLAPS special) going from intake to the decel valve. Makes for a nice tight connection at the decel. Could this affect vacuum level (more) thus causing the issue - don't see how, but just asking?

Thanks.

[schoward]

Sorry for the bump - just following up on this given the fix has been in 2+ years now and its been perfect. In short - crush fix works when done right, but my lesson learned is below....This is for the 81-83 saucer type decel valves. And sorry - I am sure 99.9% of SC owners wont care, but I had to solve this for myself.

I tested several decel valves with a hand vacuum pump including a brand new one out of the box from Porsche and all seemed to pull in at 17in hg (just like CIS Primer says). I had 3 or 4 on hand.

Per Tony's suggestion above, I also rigged a vacuum gauge in the engine bay near the oil filter secured with tie wraps and compared the vacuum on that line to the 17 in hg value of the decal valve. Turns out I was occasionally getting enough of a steady 18 in hg of vacuum on that line which held the valve open and hung up the rpms and sending it into a self defeating cycle. Whenever I would have the hanging idle, I would pull over, jump out, pop the lid and check the gauge. I probably left the gauge in there 6 months as it was tied up nicely to the side.

I found that the crush fix of the decel valve for this worked nice and altered the activation of my original valve, making it pull in at 19.5-20 hg which solved my problem. Any occurrence of 18in hg vacuum now is not enough to open it, but the valve still allows a little air burst coming off throttle during the moment the vacuum spikes over 19.5/20 in hg when I come off throttle which slows the rpm drop just a bit. I observed it like clockwork watching the gauge in the engine bay and revving the engine. Its like a modern car now. No more 'slamming to zero' RPM and no more hang/slow drop - and it seems just enough I where don't get any exhaust pops on decel. Been in there since Oct 2015 and not one occurrence of climbing/hanging rpms (on decel, idling or otherwise) while still giving me some ease down of RPMS coming off the gas.

One note - I tried crushing some of my valves with a C-clamp over and over and guess what - when I put them on the hand vacuum pump there was no change - you have to compress in a vice and not just crush the thin nipple side but compress the whole thing from both sides. Again - have to confirm you accomplished anything via a hand pump. Even the brand new decel valve out of the box from Porsche I bought tested at 17in hence the factory bulletin on the crush fix - they were wrong right out of the box. The altered valve is my original so these things can last. But you have to be careful and check your results. Not enough crush and probably wont fix it - too much and it will require too much vacuum to open (hence it will do nothing).

[john walker's workshop]

Plug the hose forever. You don't need that thing on 99.9% of cars.

[gazzerr]

I found a NOS stock one to try out and mine hangs up the revs sometimes too. I haven't tried to crush it. It pulls in at 17hg too.