CIS Surging Under Constant load

[kltarga72]

Originally Posted by massmike

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

Are you getting the pulling/surging when your driving at normal speeds and rpms are above 3K? If I drive my SC at low rpms I feel the same response you are experiencing. Drive your SC, maybe issue will resolve itself after 200 miles. See link attached
https://forums.pelicanparts.com/porsche-911-technical-forum/6523-911-driving-tips-2.html

[kltarga72]

Massmike
Came across another thread that might be helpful..
https://forums.pelicanparts.com/porsche-911-technical-forum/927421-solved-3-0sc-cis-bucking-surging-bad-idle.html

[AndrewCologne]

Originally Posted by massmike

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)

Follow my recommendation above:

Originally Posted by AndrewCologne

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).

Originally Posted by massmike

-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

With an initial CO setting where idling results in a 50–55% duty cycle, it should normally fluctuate around 38–45% while driving — but not 46–60%, as that range is too wide and is exactly what causes the surging or pulling effect.

Originally Posted by massmike

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.

First, try the other 3-wire lambda sensor mentioned above.
If that doesn’t work, the ECU might be faulty.

Where are you from — the US or Europe?

[massmike]

Originally Posted by kltarga72

Are you getting the pulling/surging when your driving at normal speeds and rpms are above 3K? If I drive my SC at low rpms I feel the same response you are experiencing. Drive your SC, maybe issue will resolve itself after 200 miles. See link attached
https://forums.pelicanparts.com/porsche-911-technical-forum/6523-911-driving-tips-2.html

Thank you, I read though that thread and tried to adjust my driving style with keeping revs a bit higher than I normally would. I honestly didn't notice much difference in the surging effect. I am still thinking something is off.

[massmike]

Originally Posted by AndrewCologne

Follow my recommendation above:

First, try the other 3-wire lambda sensor mentioned above.
If that doesn’t work, the ECU might be faulty.

Where are you from — the US or Europe?

Working on adding a 3-wire O2 sensor now. Interestingly enough while looking for a way to tap into the 12V power from the WUR I discovered this loose plug with a red/white & brown wire. This plug is showing 12V with the key on and 0v with the key off. I am not finding this on the wiring diagrams that I have, however it does look like this would be a perfect fit to this oxygen sensor.

I am from the US.

https://www.pelicanparts.com/More_Info/92860612400.htm?pn=928-606-124-00-M14&gad_source=1&gad_campaignid=22944729823&gbraid=0AAAAAD_tz51aR6fEzeMTMQXCHDvDUcXuH&gclid=CjwKCAiAwqHIBhAEEiwAx9cTeXkqAtAWK5EzQELK7Myr xnx3zIArg2uIUcFEnzW5tk1de4lQde40exoCanAQAvD_BwE

[AndrewCologne]

For the three-wire sensor, you should use power from the fuse box — the circuit that’s live when the key is on.
You don’t need a Porsche branded sensor; just use the one directly from Bosch: 0 258 986 502.

If you lived in Europe, I would have sent you a Digi-ECU to test — but with the current tariffs, and especially the extremely high shipping costs from Germany to the U.S. right now, that’s just not really feasible.

[massmike]

I did some testing and best I can tell this plug I found is getting power from Pin 30 at the fuel pump relay, so this power would be pulled from fuse #16 and passed through a relay. It seems like it would be the perfect power source for the 3 wire oxygen sensor.

[AndrewCologne]

The sensor’s heating element draws a lot of current, so as I mentioned, use the switched power from the fuse panel on the left side.

Or use the current from the wire in the pic above from the fuel pump relay for driving a relay which connects the heater to pin 30 in tje fuse box on the left.
This is the safest option as by this the sensors heating element gets only current when engine is running

Also, with a properly functioning lambda control, no specific driving style is needed. With my engine—and in many others where the lambda control is active—you get full performance across the entire rev range without any surging or similar issues.

[massmike]

Another update... I was able to add a 3 wire O2 sensor to the car. I used a Bosch 13918 which was powered with a relay. I used a positive signal from the unused thermo valve plug that I mentioned above and constant 12V came from the Heater fan fuse in the rear fuse panel. Unfortunately this made little to no difference in the surging effect while driving with light loads.

I also spoke with the folks at Systems Consulting regarding the ECU box. They mentioned that these Lambda boxes are generally pretty reliable and generally work or don't. I described the problem I was experiencing and they didn't feel that the ECU would be to blame but did offer to test it for me.

https://systemsc.com/index.htm

I am still on the hunt for the next thing to check

[930cabman]

my near original 1975 911S had the same/similar issue, I chased it for several years. Just converted her to twin Weber 40 IDA carbs. Best thing I have done in years. This beauty was basically not enjoyable to drive.

Best she has run in many many years

When new the CIS was unbeatable, after 50 years, not so much

my 1987 930 still has the CIS, runs great

[AndrewCologne]

Originally Posted by massmike

Another update... I was able to add a 3 wire O2 sensor to the car. I used a Bosch 13918 which was powered with a relay. I used a positive signal from the unused thermo valve plug that I mentioned above and constant 12V came from the Heater fan fuse in the rear fuse panel. Unfortunately this made little to no difference in the surging effect while driving with light loads.

I also spoke with the folks at Systems Consulting regarding the ECU box. They mentioned that these Lambda boxes are generally pretty reliable and generally work or don't. I described the problem I was experiencing and they didn't feel that the ECU would be to blame but did offer to test it for me.

https://systemsc.com/index.htm

I am still on the hunt for the next thing to check

IF the ECU is not at fault, then the cause of the surging is actually quite simple: regardless of the lambda control in your case the CIS system obviously does not provide a stable mixture or AFR under steady-load driving conditions. The AFR swing (its amplitude) is so large that the original ECU cannot compensate quickly or effectively enough, causing the mixture to become intermittently too lean — which results in surging. You can deactivate the regulation and drive as suggested earlier in this thread. However, that means you must enrich the base mixture in static open-loop mode so that the highest AFR peak does not exceed 14.7:1.
But as mentioned, this does not eliminate the root cause — the engine will still run incorrectly, since this would also mean that under certain operating conditions, the mixture becomes too rich. It may result in smooth driving (as you already reported), but with improper fueling and the associated drawbacks.

[mysocal911]

It's critical for the warm WUR control pressure be set properly for open loop, checked using the pressure gauge.
In closed-loop, the Lambda ECU should not need to vary the pressure significantly, causing surging, to achieve the optimum Lambda.

[massmike]

Originally Posted by mysocal911

It's critical for the warm WUR control pressure be set properly for open loop, checked using the pressure gauge.
In closed-loop, the Lambda ECU should not need to vary the pressure significantly, causing surging, to achieve the optimum Lambda.

About two months ago I had sent the WUR to Tony (on here) to have him go through it and make sure it was up to spec. The only issue he noted was the speed from cold control pressure to warm was quite fast so he made some adjustments to that. I also found a failing fuel accumulator was causing a hard start issue, this was replaced.

I can go back and verify control pressures again to make sure nothing has changed.

Any thoughts on the Frequency valve, Anyway it could be sticking, or failing?

[mysocal911]

Originally Posted by massmike

About two months ago I had sent the WUR to Tony (on here) to have him go through it and make sure it was up to spec. The only issue he noted was the speed from cold control pressure to warm was quite fast so he made some adjustments to that. I also found a failing fuel accumulator was causing a hard start issue, this was replaced.

I can go back and verify control pressures again to make sure nothing has changed.

Yes, check the warm control pressure to spec.

Originally Posted by massmike

Any thoughts on the Frequency valve, Anyway it could be sticking, or failing?

Just monitor the control pressure while hot & running. The pressure per a gauge should not vary.
Check it with the O2 connected & not connected, i.e. basically no pressure difference.

If still no improvements, one can always attenuate the O2 signal, e.g. reduce by 50% - a simple two resistor voltage divider in the Lambda ECU.

[massmike]

Another update. I took the car for a 20ish mile ride to get some heat in the engine, then checked pressures. Fuel pump pressure was measured at 67psi and hot control pressure at 49psi. Both are on the low side of the spec, but nothing major. I do see about a 1psi increase on hot control pressure if the motor is revved, but I just equated this to a slight increase in voltage to the fuel pump. Should I be concerned with either of these readings? No change in pressures from O2 sensor connected or disconnected.

[massmike]

Originally Posted by AndrewCologne

IF the ECU is not at fault, then the cause of the surging is actually quite simple: regardless of the lambda control in your case the CIS system obviously does not provide a stable mixture or AFR under steady-load driving conditions. The AFR swing (its amplitude) is so large that the original ECU cannot compensate quickly or effectively enough, causing the mixture to become intermittently too lean — which results in surging. You can deactivate the regulation and drive as suggested earlier in this thread. However, that means you must enrich the base mixture in static open-loop mode so that the highest AFR peak does not exceed 14.7:1.
But as mentioned, this does not eliminate the root cause — the engine will still run incorrectly, since this would also mean that under certain operating conditions, the mixture becomes too rich. It may result in smooth driving (as you already reported), but with improper fueling and the associated drawbacks.

It is quickly getting to the end of driving season here in Massachusetts, do you think its worth having the ECU send out for bench testing while the car sits over the winter? I am not 100% ready to give up on the Lambda system yet. Getting close though! haha

[ahh911]

"If still no improvements, one can always attenuate the O2 signal, e.g. reduce by 50% - a simple two resistor voltage divider in the Lambda ECU."

Dave, what is the purpose behind this?
Phil

[AndrewCologne]

…lets the signal provide less voltage to the ECU, and the signal’s amplitude will be lower. The ECU interprets this as a too lean mixture even if it's ok and increases the duty cycle.

It’s ok for testing purposes, but it still compensates for a fault on the CIS side, so the real solution is to fix the existing problem.

[AndrewCologne]

Originally Posted by massmike

... Fuel pump pressure was measured at 67psi and hot control pressure at 49psi. Both are on the low side of the spec, but nothing major. ...

Nope, values are not ok.
You should first have your WUR checked and readjusted. Ask Tony here in the forum — he’s your man.
Here are the correct pressures mentioned:
https://nineelevenheaven.wordpress.com/adjusting-the-warm-up-regulator/
Then take another test drive. And if you still have issues afterward, ask Dave if he can test your ECU.

[mysocal911]

Originally Posted by AndrewCologne

…lets the signal provide less voltage to the ECU, and the signal’s amplitude will be lower. The ECU interprets this as a too lean mixture even if it's ok and increases the duty cycle.

It’s ok for testing purposes, but it still compensates for a fault on the CIS side, so the real solution is to fix the existing problem.

The basic problem is the range of the fuel mixture change, based on the O2 input (pin 2 of Lambda ECU) change being too large, resulting in an excessive fuel control pressure change.
This is what causes the engine surging. An incorrect warm control pressure WUR setting can cause this, or the Lambda ECU's closed loop feedback gain.