Definition of Pneumatic valve diaphragm?

[domanows]

In the CIS troubleshooting guide published by Porsche, it states the following.

TEST CONDITIONS
"Accelerator linkage returns to idle position and moves freely"

ISSUE
"Idle sticks at about 2000 rpm"

POSSIBLE CAUSES
"Pneumatic valve diaphragm reacts too slowly"

I have a problem kinda like this. The book doesn't define the Pneumatic valve diaphragm. or how to resolve its slow reaction time.

Any clues?

Matt

[MotoSook]

A pnuematically actuated valve needs a diaphragm or piston to move the valve (other types too, but these are the most common).

Usually, the diaphragm or piston has a spring acting on it in one direction, and "gas" is used to work against the spring to cause the valve to open or closed.

In your case, it may be a punctured diaphragm or a weak spring or both or a sticky spring. Better to just get new decel valve or disable it. That's assuming you are talking about your decel vavle.

Edit: Sorry didn't answer your question, but here it is. A diaphragm is a piece of rubber sandwiched in the valve to create two separate chambers. Vacuum or pressure act on the diaphragm to move a shaft that's attached to the center of the diaphram. The shaft is connected to a valve plug or other restrictor.

[sammyg2]

Sounds like the decel valve to me.

[domanows]

I love the Pelican site! I found this image


As you know (since you've already given me a ton of great advise), I have an 83 911 SC. Where the heck is this damn thing. Maybe I am an idiot, but I only see a the cruise control contraption (looks sorta like the picture but has a few more hoses), another dongle on the distributor (has two connections but looks different, and some thing on the passenger side of the engine compartment with only one line.

Which one is it? and once I find it, I'd like to disable it to see if this is in-fact the thing causing the problem. If it is, $60 from pelican is a steal. A friend told me my fuel distributor might be worn out (over 1K yikes!).

[MotoSook]

Doman, the decel valve is at left of the throttle body toward the firewall. That's what the other big thingy in the above picture is, the throttle body. Now do I get beer?

AS for your fuel D. is it leaking? If not don't touch it yet..make it your last troubleshooting stop..run a search on "CIS" and learn the CIS system. It will help you to understand your car..understanding will rid you of the fears of owning a CIS 911. Then you can stop buying everyone beer!

GOTO: http://members.rennlist.com/jimwms/CIS/CIShome.html

very nice CIS page...and he has used parts too (if you need it), but buy from Pelican too

[RoninLB]

I think the '83 looks different.. I suggested disconnecting it very early in the high idle thread..............Ron

[MotoSook]

Hey Ron, remember when I hated CIS and wanted nothing to do with it Hahaha...I think that was a Rennlist discussion you and I had.

Doman, that wasn't so long ago, and I'm no expertnow, but I'm not afraid to mess it up

[domanows]

Souk, thanks for the link. I have been doing a ton of reading about the CIS systems. I even have the actual Porsche CIS troubleshooting manuals (I can scan and post them if they would be useful to anyone.) My biggest problem is that working on anything mechanical has always scared the crap out of me. I was always the kid that got a model for Christmas and three days later it was in the garbage and three of my fingers were superglued together
In all honesty, when I bought my car ~4 years ago, if I could have found a shop around here that had any idea what they were doing and fixed more than they broke I definately wouldn't have started doing the work myself. In retrospect, working on my 911 is very theraputic. Thanks to you guys, it has become a welcome diversion from a generally unsatisfying existence in corporate America.

Anyways, in answer to your question, my fuel distributor is not leaking, but local acquaintance (i.e. guy that comes over to smell my oil to make sure that the smell of fuel in it is only residual from last issue after changing it) stated he thought that it might be the problem. The decel valve sounds much more reasonable.

RoninLB, although this my sound stupid, how do I disconnect the decel valv? I believe my decel valve also controls my cruise control. there are two small vacuum lines running out of it and I believe one big one. I replaced all the small vacuum lines. I didn't replace the large hose. In addition to that, there is a cable that controls the cruise control coming out of it. It is directly against the driver's side firewall. Is this in fact the decel valve? And if so, if I disconnect those lines should I cap them with something?

I disconnected the cable from the throttle body, that didn't help. even when the throttle body is totally closed the RPM's increase. Am I correct in thinking that one of the small vacuum lines connecting the decel valve to the throttle body is suppose to generate positive pressure that should force the fuel distributor flapper down when the car is at idle?

Thanks guys,
Matt

[RoninLB]

Originally posted by Souk

Hey Ron, remember when I hated CIS and wanted nothing to do with it

Yep... something to do with your experience w/EFI in college.. and you invited me for coffee in Chicago. which was very hospitable.. I wound up ducking rain comming down from the Great Lakes and sliding thru St. Louis going west.. and beat up on the way home after 79 days on the road.. LOL

and Matt... I'm not saying I know what your problem is. only that removing the deceleration valve as a potential problem source is easy.. it was a smog install in days of old. It slows the rpm from falling quickly from around 2k rpm.. to remove the deceleration valve operation from the system in the posted pic calls for removing the small top hose and plug the removed hose.. I would go this route at a low idle, thus preventing unexplained periodic high idle.. or maybe the valve is stuck permanently causing high idle. I think your 911 may have a different style unit.. but it's the same mechanics.. there are a few problem sources that have to eyeballed.... maybe your dizzy advance weights are rusted /frozen, etc.. and my fuel dist. has 140k/no problem..............Ron

[domanows]

Ron, let me know if my logic regarding the deceleration valve is correct. If its purpose is to slow the RPM's from falling around 2k rpm, a malfunction on its part could either case the RPMs to drop too quickly or sometimes very delayed. Also since my cruise control is part of this system, it could affect my cruise control operation. I have always thought my cruise control was REALLY jerky and just figured that it was due to being a more primitive system. I think this gives added merit to the decel valve being the gremlin.

Also, I did a search and found posts of people discussing problems with dizzy advance weights, but no definition. I was hoping you could post a quick definition of what they are.

Thanks,
Matt

[RoninLB]

Hey Matt... I'm not a pro.. so this is my perception..

I guess if the deceleration valve was bad it could be stuck allowing a normal engine rpm fall.. the only problems I know about is when it keeps the idle HIGH.. that's when it makes you nuts trying to find the reason for high rpm somewhere in the system.. and it's an easy quick disconnect. maybe do a search for a pic for your car unit.

Cruse Control.. the deceleration valve only affects the lower rpms. and Cruse Control is usually used at mid rpms. maybe you got a bad vacuum leak somewhere.

the dizzy weights move due to centrifical force pushing them outwards against springs, controling the timing advance curve.. the dizzy weights may become stuck due to a lack of routine dizzy oiling maintence.. you can check with a timing light.. rev the engine and watch the timing move.. if it doesn't move, NG.. but check your total timing to make sure it's correct..

Cruse control.. I would disconnect it for now.. there was an upgrade to a cruse control engine bracket.. I don't know much about our 911 cruse control/don't have one.. but I think there is a vacuum diaphram control unit involved.. maybe that's leaking, causing "REALLY jerky". and the mechanical Cruse Control control of the throttle needs an eyeball..