CIS Surging Under Constant load

[kltarga72]

Originally Posted by massmike

Yes, It is most noticeable at light throttle while cruising (Typically 2K-3K rpm). If you accelerate hard the surging is not noticeable. No surging at idle either.

Massmike, I noticed you mentioned when cruising, your in the 2000-3000 rpm range. My experience with my 1980 CIS that if I do run under 2500 rpms I get a similar surging feel. Porsche reccomends 3000-4000 rpms for normal city driving and when cruising at higher speeds 3000-3500 is ok in higher gears
A porsche engineer told me many years ago to keep rpms above 2800 minimum.
Hope this helps

[AndrewCologne]

Originally Posted by pmax

Never heard of the "well known pulling" effect until your link above.

My 80 exhibits no such behavior when "cruising".

That’s because everything is installed correctly and apparently set up properly, so everything runs perfectly.

There are cases where a resulting lambda/AFR value oscillates back and forth with a too high amplitude. Normally, it moves between lambda 1.03 and 0.97, but if it goes beyond, for example, 1.05, you can notice such amplitude oscillations (around 1 Hz) as a “feels like pulling” effect.

Originally Posted by Funracer

.... None of this occurs if the O2 sensor is unplugged. ...

Unplugging the connector and running everything permanently in open-loop mode fixes the symptoms — but not the cause. And those who managed to get everything working again wouldn’t want to give up the regulation anymore.
Fun fact: I don’t know of a single test report from the years 1980–1983 that claimed the reputation of the Porsche and BOSCH engineers was ruined because the U.S. models with the K-Jetronic from those years had issues.

Originally Posted by pmax

My next suggestion ...


Did you connect that yourself ?

You might wanna install the genuine Porsche part.

Yes, connection seems done in a poor way.

Regarding the Porsche part suggestion, ... a genuine BOSCH sensor, or a good-quality alternative from another manufacturer, will work just as well — it simply needs to match the BOSCH part number 0 258 986 501.
However, when using a narrowband sensor, I strongly recommend switching to a 3-wire version with an integrated heater (BOSCH part number 0 258 986 502). Connect the black wire to the lambda connector, and the two white wires from the heater: one goes to ground (GND) and the other to switched 12 V — ideally via a relay controlled by the same signal as the WUR. This way, 12 V is only supplied when the engine is running.

Originally Posted by kltarga72

Massmike, I noticed you mentioned when cruising, your in the 2000-3000 rpm range. My experience with my 1980 CIS that if I do run under 2500 rpms I get a similar surging feel. Porsche reccomends 3000-4000 rpms for normal city driving and when cruising at higher speeds 3000-3500 is ok in higher gears
A porsche engineer told me many years ago to keep rpms above 2800 minimum.
Hope this helps

Nowadays, when society has been stirred up against cars, if you keep driving constantly around the city here in Germany at 3000–4000 rpm, the police will definitely pull you over and remind you that the law requires you to avoid high RPMs.

The rule of thumb is:
When a lambda-controlled K-Jet system is working properly, it delivers performance throughout the entire RPM range without any surging or similar. I can confirm this from my own experience and from others who have finally set up their systems correctly and, where necessary, replaced faulty components such as sensors, ECUs, switches, plugs, or wiring.

[massmike]

Originally Posted by pmax

My next suggestion ...


Did you connect that yourself ?

You might wanna install the genuine Porsche part.

The connection is a crimp style butt connector with a piece of shrink wrap around it. This was all provided with the Bosch replacement O2 sensor. Not saying this is 100% correct but it must be what Bosch recommends if they are providing it.

https://www.pelicanparts.com/More_Info/91160612300.htm?pn=911-606-123-00-M14&bc=c&SVSVSI=0574

[massmike]

Originally Posted by Funracer

Is your 35C temp switch plugged in? The switch is on the front side of the engine below the throttle body. You can reach through the fuel lines around the throttle body and feel if the wire is connected.

This switch is normally open (NO) under 35C and if unplugged (or failed open above 35C) will remain open and never close. It works with the little grey throttle enrichment switch attached near the top of the throttle body with green and brown wire. When open it adds fuel for 2 seconds every time you touch the throttle when engine temp under 35C. With each shot it throws the AFR gauge rich (lowers AFR) by 1 or 2, which is a lot. Even at cruise you are touching and releasing the throttle unconsciously to maintain a speed. It makes the gauge jump all over. You can feel the engine surge at cruise as it richens with the slightest touch of throttle.

I have unplugged mine on purpose (with O2 also unplugged) and watched and felt this surge happen. Long shot but easy to check since you have an AFR gauge.

Good luck

I tried unplugging the 35C switch (A pain to get to!) but no difference in the surging. It did seem like the it made the car start a bit harder when slightly warm (car off for about and hour). Thank you for the sugestion.

[massmike]

There are cases where a resulting lambda/AFR value oscillates back and forth with a too high amplitude. Normally, it moves between lambda 1.03 and 0.97, but if it goes beyond, for example, 1.05, you can notice such amplitude oscillations (around 1 Hz) as a “feels like pulling” effect.

What would typically cause this?

Unplugging the connector and running everything permanently in open-loop mode fixes the symptoms — but not the cause. And those who managed to get everything working again wouldn’t want to give up the regulation anymore.
Fun fact: I don’t know of a single test report from the years 1980–1983 that claimed the reputation of the Porsche and BOSCH engineers was ruined because the U.S. models with the K-Jetronic from those years had issues.

At this point I do not want to just leave the O2 senor disconnected. That's why I am trying to understand what could be causing this issue.

Yes, connection seems done in a poor way.

Regarding the Porsche part suggestion, ... a genuine BOSCH sensor, or a good-quality alternative from another manufacturer, will work just as well — it simply needs to match the BOSCH part number 0 258 986 501.
However, when using a narrowband sensor, I strongly recommend switching to a 3-wire version with an integrated heater (BOSCH part number 0 258 986 502). Connect the black wire to the lambda connector, and the two white wires from the heater: one goes to ground (GND) and the other to switched 12 V — ideally via a relay controlled by the same signal as the WUR. This way, 12 V is only supplied when the engine is running.

This is the connection that Bosch provided with the replacement O2 sensor. From what I am seeing 0 258 986 502 and Bosch 11027 are the same sensor. Can anyone confirm that? I have 11027 installed currently. I do like the idea of a 3 wire sensor with built in heater.

The rule of thumb is:
When a lambda-controlled K-Jet system is working properly, it delivers performance throughout the entire RPM range without any surging or similar. I can confirm this from my own experience and from others who have finally set up their systems correctly and, where necessary, replaced faulty components such as sensors, ECUs, switches, plugs, or wiring.

What would be the signs of a failing ECU? I have opened mine (0 280 800 055) and visually nothing looks burnt or failed but I have not yet sent it out to be bench tested.

[930cabman]

have gone through this for the past couple years, basically my near original 1975 911S has sat in the garage while my 914 and Alfa spider have gotten all the seat time.

In the process of ditching the CIS and fitting a set of Twin Webers

From what I have heard, many of these 1974 - 1985 ish CIS cars were like this from the factory

[Funracer]

Originally Posted by massmike

I tried unplugging the 35C switch (A pain to get to!) but no difference in the surging. It did seem like the it made the car start a bit harder when slightly warm (car off for about and hour). Thank you for the sugestion.

If no change the 35C switch could have failed.

Per Bentley, 35C temp switch provides cold enrichment off idle (2-3 degrees) and, here is the interesting part for you, again at 15 degrees throttle movement.

I think I was not as clear as I should have been. Unplugging the the 35C sensor will make the car surge all the time, even in cruise. With it unplugged the car thinks it is always under 35C, so you get enrichment any time the throttle is moved even a bit like in cruise. You will see this happen on your AFR gauge.

So you are trying to identify whether the 35C switch has failed open above 35C. If failed open, with it plugged in it will act the same as if its unplugged, enriching for 2 seconds every time throttle is cracked or hits 15 degrees in cruise, causing a surge.

Two ways to test. Much easier to see if the car is set to run with O2 sensor unplugged as the lambda system will counter the effects somewhat. But you can still do it with O2 plugged in if thats how you are currently set up. Just have to look a little closer at the AFR gauge.

Leave the 35C temp sensor plugged in, drive til engine above 35C then look at the AFR gauge in cruise. Push the throttle enough to open the off cruise 15 degree switch. Just a tiny bit should be enough. If the 35C sensor is working, there should be no 2 sec enrichment. If it has failed the AFR will drop instantly way down to 12 or so, you feel a slight surge, then after 2 seconds (per Bentley) AFR pop back up to wherever it was. It’s very obvious.

Other way to test 35C temp switch is with temp based continuity while in the car or on the bench. Normally open (NO) below 35C, closed above 35C.

Your comment about warm starting is unexpected. AFAIK this temp sensor works with the off idle and cruise 15 degree throttle switch. So it would never affect the start, either cold or warm or plugged in or unplugged because you don't on purpose open the throttle for start. Is your off idle switch adjusted correctly? You should feel a click just after sight throttle movement.

Anyway none of this may be relevant but when you mentioned surging in cruise this came to mind as something that could induce that.

Regards

[proporsche]

simple solution as mentioned by J.W.Get rid of the oxy sensor since you do not use the CAt..that is all you have to do
Ivan

[AndrewCologne]

Originally Posted by massmike

What would typically cause this?

- bad OX sensor
- bad ECU
- bad initial CO setup
- false air
- ... etc

I do like the idea of a 3 wire sensor with built in heater.

yes, ... faster sensor initializing and better stable sensor signal output

What would be the signs of a failing ECU? I have opened mine (0 280 800 055) and visually nothing looks burnt or failed but I have not yet sent it out to be bench tested.

Since the ECU is a complex electronic unit, there are many possible symptoms that can result from a faulty ECU.
Check out this thread where MySocal911 shows how to check the ECU pins:
https://forums.pelicanparts.com/porsche-911-technical-forum/1055502-high-dwell-911-sc-81-lambda-us-5.html

[AndrewCologne]

Originally Posted by Funracer

If no change the 35C switch could have failed.

Per Bentley, 35C temp switch provides cold enrichment off idle (2-3 degrees) and, here is the interesting part for you, again at 15 degrees throttle movement.

I think I was not as clear as I should have been. Unplugging the the 35C sensor will make the car surge all the time, even in cruise. With it unplugged the car thinks it is always under 35C, so you get enrichment any time the throttle is moved even a bit like in cruise. You will see this happen on your AFR gauge.

So you are trying to identify whether the 35C switch has failed open above 35C. If failed open, with it plugged in it will act the same as if its unplugged, enriching for 2 seconds every time throttle is cracked or hits 15 degrees in cruise, causing a surge.

Two ways to test. Much easier to see if the car is set to run with O2 sensor unplugged as the lambda system will counter the effects somewhat. But you can still do it with O2 plugged in if thats how you are currently set up. Just have to look a little closer at the AFR gauge.

Leave the 35C temp sensor plugged in, drive til engine above 35C then look at the AFR gauge in cruise. Push the throttle enough to open the off cruise 15 degree switch. Just a tiny bit should be enough. If the 35C sensor is working, there should be no 2 sec enrichment. If it has failed the AFR will drop instantly way down to 12 or so, you feel a slight surge, then after 2 seconds (per Bentley) AFR pop back up to wherever it was. It’s very obvious.

Other way to test 35C temp switch is with temp based continuity while in the car or on the bench. Normally open (NO) below 35C, closed above 35C.

Your comment about warm starting is unexpected. AFAIK this temp sensor works with the off idle and cruise 15 degree throttle switch. So it would never affect the start, either cold or warm or plugged in or unplugged because you don't on purpose open the throttle for start. Is your off idle switch adjusted correctly? You should feel a click just after sight throttle movement.

Anyway none of this may be relevant but when you mentioned surging in cruise this came to mind as something that could induce that.

Regards

Or, in a nutshell, for testing purposes just unplug the acceleration unit, and no 75% duty-cycle enrichment will occur when the 15° throttle switch position is triggered.

[proporsche]

again ,no reason to have OXY sensor in the system if you are not using the CAT....
Ivan

[AndrewCologne]

again ,no reason not to get things working the way they were engineered — especially since that’s what was clearly asked for above. Amen.

[Funracer]

Originally Posted by AndrewCologne

Or, in a nutshell, for testing purposes just unplug the acceleration unit, and no 75% duty-cycle enrichment will occur when the 15° throttle switch position is triggered.

Much easier than my idea. Thanks