what is this and is it needed

osidak · 01-22-2002, 03:25 PM

The car is a 78 Euro 911 with all emission devices removed, ie no air pump and related plumbing and if it ever had a cat she doesn't have one now.

rebuilt the engine and while hooking up all the various bits of the intake system I discovered the top (verticle) vacuum hose is block by a screw. What is this device used for and should I hook it back up?

Here is the device

The upper most hose is the one that is blocked.

Here is a photo of the engine on a hoist moving from the stand to the dolly.

Thanks for the help
Osidak
78 911sc Targa

Jim T · 01-22-2002, 03:33 PM

That is a deceleration valve, and no, you don't need it (unless you need it to pass a visual smog test, which is unlikely, since you can't really even see it).

The reason the top line is plugged is because the previous owner disabled it. With the top line plugged, it is totally de-activated.

People do that mainly because when you activate it, you often get an annoying "hanging RPM" when you take it out of gear. (Coming to a stop, you push the clutch in, and the revs hang up high for a second or so before dropping to idle).

I would remove it completely and plug the lines.

Superman · 01-22-2002, 04:00 PM

I was involved in some controversy about this a while ago, and it is still not resolved in my mind. On my car, there is a device that does not look like the one pictured. It is located over the #6 cylinder head. It is shaped much like a flying saucer laying sideways. It's vacuum line was plugged when I bought it. I was told it is a Decel Valve. When I unplugge dthe line, my idle was VERY SLOW to settle down. When I plugged it again, idle returns quickly.

Perhaps there are two kinds (and locations) for Decel valves.

osidak · 01-22-2002, 04:10 PM

My 78 has one of those as well. I will take a picture of it and post it in a bit. Mine is hooked up as of now. My idle was kinda erratic when cold but I have always been told this is a quirk of CIS.

osidak

osidak · 01-22-2002, 04:42 PM

superman here are the photos of that pancake device located above the #6 cylinder. Is this what you where speaking about

osidak
78 911sc Targa

makaio · 01-22-2002, 04:54 PM

I have both of those devices, on mine as well. Which is the decel valve?

Doug Zielke · 01-22-2002, 05:13 PM

osidak has posted a picture of the Auxillary Air Valve, not the decel valve. The decel valve is located near by the AAV. It can be identified by the small diameter (1/4") vacuum hose attached to the center of it. This is the line that is often plugged to disable it.

makaio · 01-22-2002, 05:22 PM

According to the Pleican catalog this is the decel valve.http://www.pelicanparts.com/catalog/shopcart/911M/por_911M_FUELCI_main.htm

I'm confused now! What is the pancake device?

Doug Zielke · 01-22-2002, 05:36 PM

There are 2 styles of decel valve according to the Parts and Technical Reference book. One, like the saucer Superman describes (with a small vac line out the front). And one that is shaped more like a cylinder. On my '81, I have only the saucer-shaped one.

Kevin Stewart · 01-22-2002, 06:36 PM

Jim T, i have had this hanging idle as you call it for ever and no one can figure it out, coming off the freeway to a stop at the exit ramp or any where were i had been on the gas it always idles high and can not get it to go down ill try disconnecting it, problem is the cars in storage till march, Kevin

dwil · 01-22-2002, 07:22 PM

I think that valve is to add air to the mixture
under periods of very high vaccum, ie using
engine braking down grades. It helps to
complete combustion so you don't get the
combustion in the exhaust (backfires)

Dave
77 911

retnug · 05-29-2002, 06:04 AM

I have done a lot of searching around on this subject if anyone is interested.

How I got into this was my '82 SC would "pop pop pop pop" on decel. I was told that this was no big deal but it annoyed me anyway. I started to do some digging, particularly around my decel valve. In following the vacuum hose across the back of the motor I found that it was broken. So of course I replaced it. In doing this I also noticed that one of the previous owners had blocked the hose to the decel valve which in an '82 is the "pancake shaped device" on the right hand side of the engine with the 1/4" size vacuum hose coming out the center of it.

I know, I know there has been debate about this. Here is what I have found. There were two types of decel valves in the SC years. The earlier ones '78 - '79 had the one pictured at the top of this thread which I have heard is adjustable. I don't even think this one is pictured in the Bentley's Manual for SC's. The newer SC's only had the "pancake" decel valve Which is pictured.

Anyway beck to my saga. After doing this I take the car for a driveand the popping has gone away. YEAH! I says to myself patting myself on the back. So I keep driving. All is good. I keep driving and the engine gets warmer. Still good. I keep driving and now when I come to a stop and put the clutch in the RPMs stay high and then drop. The warmer the engine the higher the RPM's stay and the longer they stay there before dropping. Now what.

More research. I find that there is a supposed fix. Crush the pancake decel valve 2-3mm. What you say? Crush a part... no way. More research. No kidding you say. This is a factory bulletin fix. Crush the decel valve just a little. Its only a part that apparently costs over $150.

Okay I says... I can do that. I put it back on the car. I'll be damned it works. Go figure.

Sorry this is so long. Maybe it will help someone.

I'd be interested in what you guys think of this and has anyone else tried it.

Gunter
'82 SC Targa

wckrause · wckrause's Garage

The earlier SC's had a seperate decel valve and aux air valve. On later SC's those two components were combined into a single component that looks like the old aux air valve, but has three hoses going to it.

retnug · 05-29-2002, 06:16 AM

Yes, two larger hoses on the engine side and one small vacuum line on the outbord side of the valve. This is the valve that gets "crushed".

Kurt B · 05-29-2002, 06:46 AM

Sorry this is so long. Maybe it will help someone.

I'd be interested in what you guys think of this and has anyone else tried it.

Gunter
'82 SC Targa

Actually, that's very interesting. I hope these other guys are reading it as well!

radcon · radcon's Garage

I'll have to check this out when I get home and can look at my car. But from what I remember, my car has two of these pancake style aux. air regulators on the passenger side of the engine. They have different part numbers, but they both look the same. And mine are slightly different than the ones pictured above in that mine have the two hose connections on the engine side of the pancake, but also have a vacuum hose port on the fender side of the regulators. However, only one of the regulators actually has a vacuum line attached to it. The other one just has an open port. I should mention that I have an '80 SC. Are there any other '80s out there like mine, or did the previous owner mess it up? Also, just how do these function? What is inside of this regulator? I don't understand how this flat pancake with two hoses attached can do anything. I can understand how the one with two hoses(inlet and outlet) and the vacuum hose works - I assume the vacuum positions a diaphragm to control the flow. But as for the one without a vacuum hose attached, how does that one work?

Rick Lee · Rick Lee's Garage

I'm getting this "hanging idle" too with my 3.2 Carrera. Engine is coming out on Sat. What can I do to fix this?

retnug · 05-29-2002, 07:21 AM

radcon,

Only one of mine has a port for a small vacuum hose. It is the valve more toward the front of the engine on the right side. The other has no port. If you have two valves each with a vacuum port and one does not have a hose connected I would suspect the a previous owner put the wrong part in for one of them. But knowing how Porsche has a tendancy to changes things mid-stream it would not suprise me if it was an original install.

Richard,

I wish I knew more about the 3.2 but I have a hard enough time with the intricasies of my 3.0. I'm not sure what they did for decel valve on those years.

Superman · 05-29-2002, 07:25 AM

I feel a little vindicated. The pancake thing with the small vacuum line attached is a decel valve. Near it is another pancake shaped thing, without a small vacuum hose, which is the AAR.

My decel valve is unplugged, because I want my idle to fall quickly when I release the throttle. I get a little popping on the overrun, but much less than before I rebuilt my engine. Perhaps I'll detach and plug the two big hoses, and see what happens.

retnug · 05-29-2002, 07:31 AM

Superman,

I thought you would.

I had the pop also. Until I hooked everything up and unblocked the decel valve vacuum hose. That is when the idle started to hang high to long before dropping. The Porsche Service bulletin which recommends crushing the decel valve a little is what solved this. The better of both worlds.

My idle now drops back to idle normally and the popping is gone. You may want to try that.