CIS Surging Under Constant load
 

Looking for some help to solve a problem I have been chasing for a few months now. Car is a 1982 911SC with a 3.0L and the original CIS system. My issue is when the car is running in closed loop there is a light surging at cruising speeds. The surging can't be seen on the tach but I find it incredibly annoying to drive it like this. If the O2 sensor is disconnect the surging completely goes away, so I believe this problem is limited to the Lambda system. I do have an LM-2 wideband connected and with the O2 Sensor disconnected I see 14.3 - 14.5 while cruising, O2 sensor connected I see it jumping between 14.3 and 15.3 at the same RPM and speed range.

I have replaced the O2 sensor and the FV relay, both made no difference. I have also adjusted the mixture several times with no noticeable change in the surging. I am thinking possibly an issue with the Lambda ECU??

Any help would be greatly appreciated.

Does this occur in a certain rpm range? with light throttle?

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.

What you're describing is most commonly due to air leaks in the CIS somewhere, and CIS is very sensitive to air leaks. However, that usually causes some unevenness in the idle too. Low throttle settings mean high intake vacuum, which sucks more air through any leaks, causing the uneveness or surging. And it goes away when you open the throttle more, because that increases air flow and reduces intake vacuum. It's also odd that it goes away when you unplug the lambda system.

However, I would start with a smoke test of the intake to make sure you don't have any air leaks.

Well if you disconnect the O2 it stays in open loop. Probably better to be in a situation where the car isn't trying to "fix" a vacuum leak by adjusting fuel mix, right?

At first I thought the same thing about air leaks so I did verify, with a smoke machine that there were no leaks. First attempt identified the decel valve vac line and the cruise control module line completely disconnected, both were corrected a few months prior. As of now I have not repeated the smoke test but do get a good drop in idle speed with the removal of the oil cap.

The problem seems to be isolated with the Lambda system. You can actually easily feel when the system goes into closed loop mode as the surging starts immediately. One curious fact is the longer you drive the car and the more heat you get in it the less noticeable the surging becomes, but it still exists.

Stock exhaust?

Would you expect there to be some drivability or idle issues in open loop if there was a vacuum leak? Also the wideband is showing that the ECU is trying to lean the mixture when in closed loop (Bouncing from 14.5 - 15.5), shouldn't it try to richen if there was a vac leak?

Sure there'd be issues, but they can be fixed by adjusting the mix. If the mix is adjusted for a vacuum leak other things are going to be weird, though.

The details of troubleshooting the Lambda system escape me, sorry, my only input is why disconnecting it might stabilize things, and also that a non-standard exhaust might have issues with the O2.

Stock exhaust, except the catalytic converter has been removed and replaced a bypass pipe. Once thing I did find a bit strange was the location of the stock O2 sensor location, it only reads one bank of cylinders. I mounted the wideband sensor a few inches past the collector.


http://forums.pelicanparts.com/uploa...1760622975.jpg

I have to throw in here. Hi Mike. My 83 with lambda behaves a bit differently than your but somehow I feel the issue is related. I have AFR gauge. With O2 sensor plugged in, hesitation on throttle when cold running (open loop). I can watch the AFR cycle between open/close as the O2 sensor heats up and the AFR changes from 13.0 (open) to 14.7 (closed) then back to 13.1 then back to 14.7 then 13.3 etc. After several minutes the O2 sensor will finally be hot enough to stay in closed loop, AFR stays at 14.7and all is good. Driving the car prior to O2 going closed loop is not fun.

None of this occurs if the O2 sensor is unplugged. With no jumping back and forth between open and closed loop the AFR rises smoothly after start and reaches about 14 at warm idle. Driving the car during this warm up period is perfect. No hesitations at all.

I have SSI’s and this puts the Lambda O2 sensor bung further from its heat source and makes it harder to reach signal temperature. For the time being I am driving the car with O2 sensor unplugged like the pre 80 cars.

Best of luck

Unplug, set idle mix @3% and have fun. Euro stylin'.

Your factory o2 sensor is in the incorrect location. That location won't give the lambda system a chance to properly meter the exhaust gas. The stock location for lambda CIS cars is around where your wideband is installed.

That isn't a stock exhaust.

http://forums.pelicanparts.com/uploa...1633914966.jpg

As someone pointed out, the OEM sensor is in the wrong place.

Try swapping the two and see if it makes any difference.

I had my fingers crossed for an easy fix but....no such luck. I swapped the Lambda O2 sensor to the port farther down stream and no noticeable change in the surging while driving in closed loop. AFR readings on the wideband seems to jump all over the place when in closed loop, sometimes bouncing .8-1% in the matter if just a second, almost like the lambda system cant find the sweet spot.

In this article regarding Lambda ECU's it is mentions there is a well known "Pulling" effect while constant driving/cruising. I haven't been able to find any more information on this well know effect, however it does seem to resemble the issue I am seeing. Anyone know more about this?


https://nineelevenheaven.wordpress.com/optimized-control-unit-ecu-for-the-911-sc-with-lambda-control/

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

My 80 exhibits no such behavior when "cruising".

The bucking bronco parking lot effect, yeah BTDT and that's after paying a pro to service the system.

Bummer was hoping it would be an easy fix. Next would be to test the O2 sensor to make sure its reading accurately. Or you can just replace it with a new one. There's quality replacements for not much money. I'd test to make sure that's the case first. I'd also recommend a smoke test of your airbox and intake system to rule out any leaks.

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

My next suggestion ...
http://forums.pelicanparts.com/uploa...1760722997.jpg

Did you connect that yourself ?

You might wanna install the genuine Porsche part.

http://forums.pelicanparts.com/uploa...1760722935.jpg