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AndrewCologne 09-13-2016 08:48 AM

CIS removed - got some questions
Hello all,

a good friend of mine got some problems with his 911 2.7 from 1975.
The exhaust gas recirculation has been already removed in the past.

The idle is very, very rough and furthermore backfiring makes the engine unable to run.

What we tested:

- System pressure = ok
- Control pressure = gauge also here says its ok but hard to check as the idle is almost not possible
- All injection jets/nozzles do output the same amount of fuel
- Valve clearance is ok
- Ignition timing ok
- Ignition points = ok

So I thought lets do the whole job and remove the CIS to check all plugs, rubber, pipes etc.. Beside this its good for cleaning everything and replacing the known three seals on top of the engine.

After removing we recognized that the removal made sense as the connection of the crankcase ventilation tube was already "a bit" leaking. But ... for sure thats not the reason why the engine won't run normal without backfiring.

Then I saw a very strange type of tube connected to the black air housing below the throttle unit.
And that tube is ... open, and beside this metal tube there are two disconnected hoses as well.
See my pictures below. So ...

What was this tube (red) and these hoses (green) good for? I think we should seal/close them as this could result in a big problem with false air, correct? And so maybe that rough backfiring idle comes from?

Also after the CIS was removed there where a disconnected dirty electric plug (Plug 2) lying around.
Do you know where this Plug 2 belongs to? It seems odd to me that it wasn't connected.

Many thanks for your help!

timmy2 09-13-2016 09:33 AM

Need the colors of the wires in the plugs to correctly identify them.

75 911s 09-13-2016 09:35 AM

Hi Andrew,

The pipe coming off is the EGR valve pipe. The two vacuum lines also belong to the EGR valve.

From my understanding, exhaust comes out engine into the thermal reactors, then into the heat exhchangers, then the muffler which then some of it goes into the EGR filter, which then goes up to the valve, and back into the intake via that metal/rubber line.

Of course the thermals are probably you have 3 into 1 Bursch headers that go over to the stock heat exchangers? or do you have backdated SSI style heat exchangers that have the heat boxes and headers all in one?

The other plug I'm not sure. Did you disconnect the heater blower motor or was it intact when you took the CIS off?

I'm skeptical of the EGR being the culprit for your issues. When you get it put back on and sealed up, if you still have the problem, I would check the WUR, and then the mixture as it may have been messed with making it too lean or rich.

The mixture is super sensitive from what I've read. I wouldn't mess with it unless it's a last resort.

75 911s 09-13-2016 09:44 AM

Also, does your throttle lever work next to the E-Brake? You have to pull that so that idle is at 2k upon first start, then the WUR kicks in and the idle drops to about 1500, then you drop the lever and idle goes to a normal 900ish

Here's a pic of where the vacuum lines and EGR intake tube go

boyt911sc 09-13-2016 10:18 AM

Electrical plug connections.........

Originally Posted by AndrewCologne (Post 9279110)
Hello all,................................

Also after the CIS was removed there where a disconnected dirty electric plug (Plug 2) lying around.
Do you know where this Plug 2 belongs to? It seems odd to me that it wasn't connected.

Many thanks for your help!


This is where the plugs go:

Plug #1...............AFS (air flow sensor) 2 wires (1 brn, 1 brn/red).
Plug #2...............AAR (auxiliary air regulator) 4 wires (2 red/wht & 2 brn).
Three-wire(?).......CSV (cold start valve) 3 wires (1 red/wht, 1 yell. & 1 yell/blk).

Plug the EGR hole under the air box and the two (2) vacuum hoses too. You don't need them anymore since the EGR has been removed. Keep us posted.


timmy2 09-13-2016 11:12 AM

No AFS on '75, started in '76.
Likely the CSV and AAR only.

boyt911sc 09-13-2016 11:45 AM


Originally Posted by timmy2 (Post 9279310)
No AFS on '75, started in '76.
Likely the CSV and AAR only.

Mistook it as OP's SC. You are indeed correct CSV and AAR only. That's why I have a (?) for the three (3) wires in his picture. Thanks for catching the error. I actually checked and compared them to your new wire harness (Paul Z's) sitting in my basement.


timmy2 09-13-2016 03:34 PM

No worries Tony,
Wire colors and configurations changed year to year back then. Probably only one yellow and a red/black to the CSV connector. The yellow is likely Doubled back from pin one in the 14 pin connector over to the TTS and then to the CSV as a single yellow.
They weren't consistent at all as they changed and learned the best ways to do it!

The reason I asked for the colors instead of telling him what they should be is to determine if the correct year harness is installed!

The joys of hand built cars and previous owners. :)

AndrewCologne 09-14-2016 12:29 AM

HI all,

first: thanks for all your feedbacks - I really appreciate it.

- We will make some other pics from around the whole CIS, so you can see exactly where Plug 1 was connected to
- We also will have a look at the wire colors of the Plugs and will tell you
- This CIS doesn't have an Auxiliary Air regulator/valve - and the car has been build in June 1975, maybe thats why

So it seems we are dealing with a very early type of CIS

Best regards

timmy2 09-14-2016 08:37 AM

Ah, learned something today. The car is a euro version, so it doesn't have an AAR.
The engine harness would have been wired for both US and Euro markets, so it would have the AAR connector installed, and not be used. (Porsche only built one style of harness each year, they just didn't connect what wasn't needed)

AndrewCologne 09-25-2016 11:00 AM


this CIS here belongs to a US 911 2.7 model, not Euro.

I made some new pictures.

Below you can see the colors of the wires from each plug.
(Plug 1 gots yello-beige and red, plug 2 both in brown)
Maybe with that you can say where the other plug 2 belongs to.

Here below you can see from which port plug 1 was pulled from

Also above there are two parts where I would like to know if "theres missing something" and "what was that part good forô where they applied a plug and the the rings.

Also I would like to know to which part of the CIS this hose goes to ?

And last but not least we checked the air box where I thought the main issue/problem of the uneasy idle comes from and the hole very below which wasn't closed/sealed IMHO definitely needs to be closed as otherwise it would result in a mega false air input.
For closing/sealing used a plug normaly used for radiators which fits perfectly.

So we only need to know the infos of the plugs and the hose above, then we will re-install the CIS and .. see if the "false air" issue above was the guilty one.

BTW: WUR also works perfectly: pressure ok and the restore shows about 37 ohms IIRC.

Thanks a lot for your assistance and BR

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