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

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.

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.

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.

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.

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

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.

What would typically cause this?


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.

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.

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.

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

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

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

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

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


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

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.

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

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

Much easier than my idea. Thanks

Is this enrichment done through the cold start valve?

Any chance the surging could be coming from the O2 sensor coming in and out of its designed heat range due to the increased exhaust flow? I thought I had previously read that the O2 needed to maintain 600 degrees to produce an accurate output voltage. I have an old Cat (Unfortunately most of the catalyst has been broken) that I could install in place of the bypass pipe.

Ok so had a bit of time today to play around with a few things.

First the 35 Degree switch. I was able to reach in the back an clamp on a test lead to the electrical connection. With the car cold I had no continuity from the connection to ground, after the car was warm I did have continuity to ground. This would make the switch normally open, which I believe is correct.

Second, frequency valve duty cycle. I have played around with this in the past and have always had mixed results. Figured I would try again. Took the car for a short drive to get some heat in it, surging started once the 15 Degree switch was activated, Heading back home to connect a scope. On initial connection the duty cycle showed a pretty steady 48% (Seems like this is the open loop default), after 4-5 min of idling, I was getting some fluctuation from the duty cycle, dropping to around 30% and bouncing back to 48%. If I added a bit of RPM while watching the duty cycle you could actually hear the engine fluctuating up and down as the duty cycle changed. I tried to make small adjustments to the mixture screw but couldn't get the duty cycle to change much at all. In the end I left the O2 sensor disconnected and set idle AFR to around 14.5.

Third, I disconnected the O2 sensor and verified voltage coming from the sensor with a volt meter, I was seeing a pretty steady .8 VDC. I was expecting more movement.

It seems the initial setup of your system is off — follow the instructions here:
https://nineelevenheaven.wordpress.com/the-911-sc-3-0-engine-with-catalyst-and-lambda-control/
Don’t just adjust the mixture screw — the resulting duty cycle at idle should fluctuate around 50–55%, and while driving at higher RPMs it should fluctuate around 40%.

Everything is explained in the link above.

f the sensor is disconnected, the regulation is off, which means the lambda is far from the sweet spot where the voltage would normally fluctuate. In your case, it’s steady at 0.8 V because the initial mixture setup is too rich.

No – the mixture regulation of the lambda control is done only through the frequency valve.
A duty cycle of 100% means the valve is fully open, resulting in lower pressure in the “lower” chambers of the fuel distributor, which allows more fuel to flow from the upper chambers to the fuel lines leading to the injectors.
A lower duty cycle results in less fuel flowing to the injectors.
(all explained in the link above) :-)

Quick update here. I have read each comment and the links that have been posted. With what I read I understood that I could adjust mixture using the duty cycle of the FV, so I attempted this yesterday. What I found was the duty cycle was very inconsistent, One moment it would be 30%, then jump to 90% and then back to default at 48%. I made a small jumper wire (inserted between O2 sensor and O2 plug) to read voltage of the O2 sensor, it was rock steady at approximately .5Vdc. From there I removed the oil tank cap to simulate a lean condition, O2 voltage only dropped .05Vdc, which didn't seem right. I then removed the O2 sensor from the car to bench test. Volt meter attached to the O2 wire and the sensor body, heat applied to the sensor tip with a small torch. During the first test the voltage came up after 30 second of heating to around .6Vdc but was extremely slow to react when the torch was moved off and on of the tip. During the next test the sensor failed to produce more than .08Vdc even after 2 min of heating. I have concluded the oxygen sensor has failed, even though it was a brand new Bosch. A new sensor has been ordered.

https://youtube.com/shorts/d2Nzejd7XoA?si=PT97gxWynTyQakP2

Still hoping for some help here. I made the following changes but still have a slight surging while in closed loop.

-Replaced the O2 sensor with a new Bosch 11027 (This is the same as I previously had)
-Removed the test pipe in the exhaust and replaced with catalytic converter. (The cat only has a single cell in it, the other two were removed by a previous owner)
-Noncontact temp gun shows cat at approximately 500 degrees F at idle.
-Verified duty cycle at idle, set to 50-55% (I noticed the DC is much more stable at idle and easier to adjust)
-Verified O2 senor voltage at idle, swings from .1-.9Vdc
-Drove the car and monitored Duty cycle and O2 voltage. Duty cycle swings from approx. 46% - 60%. O2 voltage swings from .015 -.8Vdc

I am still getting a surging at constant loads, light throttle, and slow driving (parking lots).

Any thoughts on what to look at next? Again, surging goes away when system is in open loop.

https://youtube.com/shorts/e4H-zMOyLqk?si=ZqGAitLIlrR0C2CV