Lambda System functioning ?
 

Well, after a week of Fuel pressure testing and getting my feet wet on the 1981sc Lambda CIS system, I find myself wanting to verify that my Lambda system - from 02 sensor to ECU to FV etc. is working as it should.

When I purchased the car two years ago, I discovered that the ECU was not connected. Since then, the motor has been rebuilt, installed and "dialed in" by a shop that advertises itself as Porsche specialists. They installed a new 02 sensor, reconnected the wiring harness and stated all was working fine. While the source of my idling and start problems may indeed lie elsewhere, it has been advised that after verifying cold, warm and system pressures, it is now time to verify all is well with the Lambda system (I have a digital WUR installed, and have been working with the company owner to verify IT'S proper operation up till now... ).

My question is what is the best way to go about this systematically. Start at the ECU side or the 02 side ? What are the best test equipment and procedures to employ ? So far I have changed out the 02 sensor relay at the ECU, checked to be sure that the 12 (13?) pin harness connectors are installed and engaged, disconnected and reconnected the FV connector and examined the 02 sensor itself, and it's connection to the pin connector inside the engine bay (see photo). In doing this, I did notice that the 02 connector as well as it's wire leading to the harness had both seen better days. It looks as if the aforementioned harness lead has chaffed a bit over the years (although it may still be functional as it is ?). Also, the plastic surrounding the wire at the pin side, looks damaged (the pin has been removed due to looseness, and will be soldered on again if this unit is deemed usable).

So, the questions are, 1. Should I be looking at replacing the above mentioned connector and/or lead ? and 2. How should I go about my testing procedure to verify that everything in this Lambda system is working ?

While I am a relative newbie to the CIS system, I have done a bit of research on it including reviewing the Primer on the system, a number of Pelican posts and the Tech BBS on the subject. I have a voltmeter, vacuum pump/gauge as well as a CIS tester at this time (I also have a determination to understand this magnificant bastard of a fuel system, and get it all on the same page with itself !)

Anyway, all comments, advice and suggestions are thoroughly appreciated !:)

Cheers all, and thanks you.

[img]http://forums.pelicanparts.com/uploads24/02+sensor+connector+and+wire1413359670.jpg[/img

You need to connect an analog dwell meter to the test port under the plastic cover in the engine compartment to "see" what the lambda system is doing under differing conditions. Everything you need to know is in the archives, try searching for CIS lambda. Stumbling and stalling after cold start is usually caused by weak mixture from vacuum leaks, the vacuum retard and using other than BOSCH normal spark plugs. The O2 sensor is irrelevant to cold starts but the ECU and pulsing FV are not, they must be at the default duty cycle.

Thanks Paul.

I have not used a dwell meter before. Where might I find one ? Will doing this test tell me whether the ECU and FV are working and pulsing correctly ? If they are, do you suspect an unmetered air leak(s) ?

And by the way, after a reasonably good cold start and 1,200 or so idle for about 40 secs, the motor then slows to 950 or so for another minute and then will slow and stall if not throttled until fully warm. It then idles fine (with a very regular +50 rpm every couple of seconds). After 20 mins of sitting, the car will not idle at all again without throttling, until once again fully warm. It needs to sit until fully cold before once again cold starting as first described.

Best,
Bill

i would want to know what the fuel pressures are.

check the freq of the FV as said

if i could, check the AFR's

Frequency Valve
 

Have you verified that the frequency valve is working? It has a very pronounced buzz when functioning. It doesn't work till the engine is running or you jump the fuel pump relay, ignition key on. A brief search will give you the procedure for jumping the relay. I just use a couple roofing nails and a alligator clip jump wire. I think you jump 87a and 30. I always double check my reference first though.

If your car was running with the ECU unplugged, they may have richened the fuel mix to compensate for the lack of cold start enriching. Then as it warms, the engine is way too rich.

Just a thought.

Also check out Jim's "CIS Primer". It covers about all of what ails us.

Note to self, RTF post.
 

Sorry, I see that you already read that!
SmileWavy

Bill,

My advice would be to borrow one, anyone who works on cars with points will have one lying around. It sounds like your car goes into closed loop when warm , but why guess ? My advice would be to first disconnect and unplug the gray vacuum retard hose at the back of the distributor and reset the hot idle to 950 rpm with the large thumb screw on the throttle body. This usually greatly improves start up issues. If you have gauges, check the pressures against the manual specs, but a CIS lambda car can have perfect pressures and run like crap. Check the ignition timing with both hoses plugged, 5 BTDC @ 950 rpm. Then drive the car for 10 miles and check the hot closed loop dwell.

One simple test with engine fully warm is to use a good digital volt meter that can read well in the 0-1vdc range. Then simply measure the voltage at the O2 sensors signal line. If the system is functioning the voltage will swing between .2 and .8 vdc if you can clearly see this swing in voltage the system is working. If it reads a constant less than .5vdc the engine is running very lean and the system can't richen it. If it reads constant more than .5vdc then it's running rich beyond what the system can do.

And if you can get a oscilloscope you can really see that the signal is a sine-wave that swings from .2 to .8dc a few times per second. As the sensors age they respond slower and the swing takes much longer.

In Summary; measure the sensor's voltage with engine warm and report back.

Thanks all for the very valuable information.

I like to do the easy, simple things first so I will be checking for the buzz of the FV first, then move on to some basic voltage reading to verify duty cycle.

Paul, check me if I am wrong, but with the Electromotive HPV-1 distributorless ignition system that I am running, there is no vacuum advance. As you might know, advance is set in three straight line and additive rpm ranges (up to 1000, up to 3000 and up to 8000) there is also an over rev limiter setting. Mine are set at 5, 19 and 6.

Hey Bill,

Sounds like your lambda SC is far from stock, digital WUR and crank fire ignition ? You are probably by yourself with that combo. No vacuum advance or retard unless you have a MAP sensor. If I read you correctly, you are using 30 BTDC timing after 3000 rpm which is 5 degrees more timing than stock. I would be using the highest octane fuel available and be listening and looking for signs of pinging. If you have good fuel and the motor does not run hot, you may be able to get away with it, but the small port SC is tuned for a low rpm torque boost and is very knock sensitive.

How did the pressure readings compare to the warm up curve in the factory manual ? The dwell meter will show you how much the system needs to alter the lower chamber pressure to get to closer to stoich at idle. Voltage at the O2 sensor will not give you the information you need to make mixture adjustments at the FD. If you still have the stock throttle switch, the system goes open loop and ignores the O2 sensor after 35 % throttle. What controls your digital WUR ?

Thanks Paul,

The WUR powered just as the stock one was. It has a MAP sensor included in its digitally controlled motherboard. It has an electrically actuated injector that mimics the original WUR injector that was controlled by the bimetal strip. My system pressure is 4.6 bar and cold control pressure is 2.2 bar. I am working with the WUR manufacturer now to program the correct warm pressure cycle but that is another issue.

As for timing advance. Up to 1000 rpm I am at 5, the system then advances evenly up to 5 + 19 = 24 at 3000rpm. It then rises 1.2 degrees per each additional 1000 rpm up to 8000. Since I do not plan (and have a rev limiter) to go beyond around 6,800 rpm, I believe the advance should be limited to a total of 24+(3.8 x 1.2) = 28.5 total advance. At least, this is if all is operating as it should. I have 91 octane here and am always listening for detonation. One thing in my favor here is the relatively high power output of the crank fire coils. Am I wrong in thinking that this should help mitigate detonation some ?

Anyway, this is getting a little off track ....

Bill,

I don't think detonation is mitigated by spark energy and the coil only supplies the required voltage to jump the gap. The stock CDI has more than the required voltage. I have run 28 degrees of timing on an SC, but you need to watch for detonation.

Ok, if you are new to lambda CIS on a 911, here is the super condensed, almost incorrect, but necessary way to think about it. It is a basic CIS system that is calibrated too lean everywhere, that uses the FV the richen up the mixture everywhere, using a default FV pulse open loop and above 35% throttle and the O2 sensor feedback loop below 35% throttle. If the ECU loses power and/or the FV stays closed, it is too lean everywhere and it cannot be correctly adjusted out. If your pressures were in spec and the FV was closed it would run like crap. In other words, the limp home settings are just the opposite of what you would expect from BOSCH, too lean, but at least it limps with a failed ECU. Also when reading duty cycle, higher is richer when in open loop, but high is leaner in closed loop. This is because in closed loop, you are looking at the systems reaction to the O2 reading, a high reading means the system is trying to adjust a too lean condition. For a richer open loop WOT mixture, you want a lower closed loop duty cycle. OK ?

The original cold control pressure spec was 1.4-1.8 bar at 10C rising to 3.4-3.8 bar at 40C (2.2 bar is around 20C). Warm pressure should be 3.4-3.8 bar, do you have that ? System pressure is 4.5 -5.2 bar. Have you tested how quickly your control pressure rises against temperature ?

I like super condensed .... Just so that I understand, when you refer to open and closed loop, are you equating that to a functioning FV and a non functioning one ?

After monkeying with all of the harnesses and connections, I do in fact hear the FV buzzing (I do not believe this was the case during my initial fuel pressure testing). I unplugged it and tested the voltage to it and it was steady at just under 12v.

At this point, I am reconnecting the 02 sensor connection in the engine bay, because (as I understand you?) the ECU needs feedback from the sensor (at idle and below 35% throttle) to properly enrich the system ? Of course, this would be with a running motor correct ?

So far, I have done the aforementioned tests with the FP relay socket jumped at terminals 30 and 87a. As such, when I pulled the FV valve connector, I measured the voltage as a steady reading that was just under 12v. Is this normal ? Should it be at a lesser duty cycle even with a non running motor ?

Next step/test ? Thank You

Bill

Some further results ...

I repaired and reconnected the O2 sensor connector in the bay and retested the FV voltage and it was still pegged at around 12v. Perhaps with a running car and an active 02 signal, this voltage would drop to a more appropriate duty cycle (if I understand things correctly ....) ?

I then started the car with a buzzing FV (do not believe this was the case previously, when the folks who installed and "dialed in" this newly built motor 2,500 miles ago). I should mention that - upon other things - the installer had a non-functioning MAP hose connected to the WUR as well as the likely non-functioning FV and "dialed in" the mixture accordingly .... I now believe I am dealing with a different animal.

Once the car was warm ( I had to increase the idle a quarter turn to keep it idling during warm up), my control pressure was at 3 bar (I know, low) and my 02 sensor output voltage - yes, I remember Sals sage advice - stayed at around .1 volt at idle and rose to a steady .4 volt when I gunned it.

I am wondering what my next steps are ..... continue testing the ECU side of things or focus on the WUR pressure issue.

Thanks to all, I could not be more appreciative of everyones input .

Bill

Bill,

You are shooting in the dark without actually testing the duty cycle. You need to determine if the ECU is pulsing the FV in the correct range open loop and dithering in closed loop. The open loop operation (cold and WOT) cannot be tested at the O2 sensor. If your new O2 sensor produced 0.1 volt with a low 3 bar control pressure, about all you can say is the sensor is not broken and it is lean when it should be rich. Is it lean because the ECU is not pulsing the FV correctly ? The unheated narrow band sensor in an SC is not very accurate away from 0.5 volt and should be viewed as a switch. Have you tested the main temperature switch on the right timing chain cover ?

I like super condensed .... Just so that I understand, when you refer to open and closed loop, are you equating that to a functioning FV and a non functioning one ?

Bill,

Missed that one. No, closed loop is when the mixture is controlled by the O2 sensor input. The "closed loop" is, fuel injected-combustion-exhaust O2 content measure-fuel injected adjusted, lather, rinse repeat and so on. Open loop is when the amount injected is not altered by the O2 sensor, it is a default value from a table.

The key point many people do not understand about CIS lamdba is that the FV must be pulsing all the time, under a fixed default value(s) open loop and dithering around stoich when in closed loop. The goal with your motor should be optimum open loop AFR under WOT, decent cold start, then let the lambda pull the mixture back to 14.7:1 for low load part throttle cruise (85% of road driving time) .

You are probably measuring average voltage at the plug which really doesn't tell you anything.

Thanks as always, Paul (and anyone else).

Decided to invest in a new digital volt meter (Klein tools MM200) that has a duty cycle dwell and % function. Jumpered the FP relay socket at 30 and 87a (key at on) and checked the 3 pin test connector in the bay. The green and while lead pin checked in at a steady 50%. As it was 75 degrees F at the time, I checked the 59 degree F temp switch for open state by using a continuity test light on the disconnected tab (correct test ?) and there was no light as expected. I also verified my cold control fuel pressure at this time was steady at 2.2 bar.

I then started the motor (with some difficulty, but it did idle). The control pressure stayed at the 2.2 bar reading until jumping quickly to around 3bar after around 40 seconds and stayed there throughout the next 10 minutes of idling/warmup.

When the temp (at the left chainbox cover) was around 160 degrees and the motor still idling, I disconnected the 02 sensor connection and tested the duty cycle again and it was still pegged at 50%. I also (unlike last test) disconnected the 02 sensor connector from the sensor itself and put my new dvm to the sensor lead itself. Nothing .... no voltage at all. I checked on another circuit to be sure that my meter was functioning correctly and it was. The wires seem well connected to the actual sensor, and it was new 2,000 miles ago so what gives ?

I am tempted to remove and replace the sensor, but I am not sure if there is a good way to test the thing (and repair ?) first. Also, what sensor is the best one to replace it with (brand, wide or narrow band etc.) ?

At least I am making some progress ....

Best,
Bill

Bill,

The main switch is normally closed and opens when hot. The Bentley manual has this backward. The 59F/15C refers to the temperature of the engine, not the air. The factory switch has a round terminal, but some cars have the "upgrade" higher temp superswitch which opens at a higher temp. It has a flat terminal.

If the O2 sensor is not producing a valid signal, the system will not go into closed loop even if the main switch works. The sensor may be carbon fouled, you can take it out and clean it with a torch. You cannot use a heated or wide band sensor with CIS lambda, just the single wire BOSCH, which can be found for $20 if you are willing to splice the wire. The shielding is necessary, it is a long way for 0.1 volts to go. It sounds like you verified the ECU is pulsing the FV at the default setting. It also sounds like your digital WUR is wacky and raising the pressure too soon, do you have the original WUR ?

Well .... I bought the car with the CIS mods already on it so while I do not have the original dizzy and WUR, I did have the forethought to buy another pelicans used WUR (same year U.S car) just in case. I had it tested by Tony on this forum, and he concluded it needs a refurbish. Since I was waiting on a brand new digital WUR at that time, I decided to save the refurbishment for later .... I have been told by the DWUR tech that once I have the rest of the CIS system components functional again, it should be an easy software update in order to produce the desired cold - to - warm control pressure and rate of delivery defaults on my unit. Why this was not done by the installer who assured me of his ability to "dial me in" (and proved it by his premium priced invoice ), I can only guess.

By the way, I mentioned previously that the WUR did not have a functioning vacuum hose connected to it (the one the installer connected had no active vacuum), but that I had found one just below the fiber jacketed one that he used, that did have a vacuum that I tested to be around 15-20 bar up to around 2 to 2.5 thousand rpm, and then dropped to zero. Does this sound like the kind of vacuum input I am looking for for a typical WUR application ? It is the one that is connected now.

As to the 15 C temp switch, mine is gold colored and has a flat tab coming from it as a connection point for the wire. Are you saying that at the cold temperature (I measured it by shooting a laser thermometer at the base of the switch) of 75 F , that the light on the continuity tester SHOULD have lit when tested at the disconnected switches flat tab ?

Also, am I correct in assuming that the lack of a readable voltage measured at the 02 sensor was probably not because the mixture was set at a ridiculously lean mixture, and that it was more likely malfunctioning ? I also did not see any shielding whatsoever on the lead running from the sensor to the connector .... just a small diameter black wire. And yes, the same folks installed the new sensor at the same time they installed the digital WUR.

Lastly, when I do get this digital WUR programmed, what would be ideal cold and warm - and rate of transition between the two - control pressures ?

Fun, fun, fun ....

Thanks !
Bill

Does this sound like the kind of vacuum input I am looking for for a typical WUR application ? It is the one that is connected now.

Bill,

Your engine's original WUR had no vacuum enrichment, it was a vent hose, so you are throwing another variable into the mix.

The BOSCH lambda switch usually has the temp stamped on the side of the hex, I checked a few and the terminals came both ways. The switch is grounded when the engine block temp is below this number and opens above it. The idea behind the alternate switch is to delay the onset of closed loop so that then engine gets a bit more enrichment until it is warmer. There is also another ECU , throttle switch and 35C switch on the breather to provide enrichment during cold driving. Have you tested those ? All of these gizmos were to squeak through the emissions test and still have reasonable drivability when cold.

Use the factory WUR pressure/temperature chart in the manual and compare the gauge reading with your IR thermometer.

You can in car test any narrow band O2 sensor, here's how:

Disconnect the sensor from the harness so it's not plugged in at all. If the sensor has a signal line and also heater lines then if possible leave the 12vdc heater lines connected but not the signal line. Then with motor fully warm and running at idle measure the voltage at the signal line it should read somewhere in the range of 0.0 to 1.0 volts. Where anything below 0.5 is lean and above 0.5 is rich.

You won't know what side the engine is running at (lean or rich) before hand but if the sensor reads 0vdc it's either bad or very lean, so what you will do next is richen the mixture intentionally. In the CIS cars this is easy, just slightly push up on the air plate in the air meter funnel, this adds fuel to the mixture and the sensor MUST saturate at above 0.7vdc

If the sensor is already reading rich then go the other direction and lean it by pulling down on the air plate slightly and the sensor must drop below .3vdc

In the CIS cars it's very very easy to alter mixture via this air plate method.

If you find that the sensor does not respond to the mixture changes then the sensor is bad. Once you do this method on a properly working sensor you will really get the hang of how it's done via this very easy method.

That makes good sense Sal, thank you for that info.

A couple of things before I dive back in ...

Paul, are you sure that my original WUR did not have a vacuum sensor ? the primer I read on the subject gave me the impression that they added one in the mid-late seventies ?

Also, I stated that the vacuum line coming off my manifold that is now connected to the wur was pulling 15 or more bar and then dropping to zero when rpms rose beyond 2,500 or so. That was in inhg, not bar (sorry !) It is more like .6 bar ....

cheers

CIS component........
 

Bill,

Your '81 SC USA model's WUR is 0-438-140-090. This particular WUR is not vacuum assisted. If you know and understand how your CIS works particularly the fuel pressure system, using an after market digital WUR is acceptable. Provided you know how to calibrate this unit. Using an OEM WUR that is correctly calibrated would be much simple. If after installing your new digital WUR, and you are still having some sort of problem, contact me and I'll lend you one to use for your test.

Tony

Paul, are you sure that my original WUR did not have a vacuum sensor ? the primer I read on the subject gave me the impression that they added one in the mid-late seventies ?

Yes, full throttle enrichment is handled by the ECU and the hose to the WUR was essentially an overflow tube like on a marine fuel pump. There is very little accurate information on your CIS lambda on the web.

OK and Great. Thanks for the feedback Paul and Tony (as well as your kind offer).

Things are starting to make more sense to me now as little by little, both the lambda cis and this digital wurs operation is better understood.

Last night, I installed the new 02 sensor (old one was completely covered with a hard black surface) and tested things again.

This time, I was able to get a signal from the sensor lead while warm, and it was around .85 volts when engine was fully warm and at idle. At this time, the control pressure was steady at 2.9 bar. The FV still runs at 50% duty cycle in open loop, but when the sensor lead is reconnected with a warm motor, it is now around 88%.

Seems odd ? if I understand things right, the 02 sensor is indicating a rich mixture, the control pressure is producing a rich mixture, but the FV duty cycle is responding in a way that would indicate it thinks the mixture is too LEAN ?

The way I understand it, the duty cycle on a pulsed ground FV would be the INVERSE of what you would expect. It would read "on" when the FV was not receiving a ground and "off" when it is receiving a ground. That would mean that your "real" duty cycle for the FV being on is 12%, which holds hands with what everything else is telling you. Easily verified by leaving the meter hooked up and removing the oil tank cap, which will introduce unmetered air into the system and lean out the mixture, so your duty cycle should go "up" (down on your meter).

Regards,
Brian Petry, Springboro OH

Thanks Blpetry,

Well, that would surely be great news ..... I would then seem that once I increased the control pressure to specified values, the FV would then pulse on at a higher frequency and richen the mixture nicely.

But, is it true ?

Thanks very much,
Bill

Bill,

When the system goes into closed loop, the duty cycle dithers back and forth around 10 degrees, say 45-55 or 35-45. If your system is not going into closed loop, it could be the O2 sensor signal (at the ECU with your broken wire), the main switch, or the throttle switch. What is your idle speed like after a 10 mile drive ? Is it rock steady or does it dither up and down slightly ?

Your control pressure is still too low.

The reason I suggested an analog dwell meter is that you can see the quality of the sensor input and the response in the rhythm of the needle swings. A good quality analog meter is a $10 tag sale item. A tight engine with good even compression and a new sensor has a very different pattern than an old motor with vacuum leaks and an old sensor. Look in the archives, other listers have had problems with digital meters that display an averaged reading.

Decided to run a little experiment this morning to confirm what I was thinking last night.

Hooked my car (warmed up, running in closed loop) up to my inexpensive amazon oscilloscope I purchased as a result of advice here and it showed the following ~50% duty cycle (The "up" and "down" portions of the square wave are roughly equivalent).

http://forums.pelicanparts.com/uploa...1413732826.jpg

Next I removed my oil cap to introduce a lean condition and the duty cycle went "down" (the "down" portion of the square wave is now much bigger than the "up" portion).

http://forums.pelicanparts.com/uploa...1413733046.jpg

Based on these findings, I think its safe to say that the duty cycle you are seeing is probably correct but it is truly the inverse of the effective duty cycle. Your engine is running rich and the computer is responding by reducing that effective duty cycle of the FV as you suspect.

BTW - here is the oscilloscope I have. Its very inexpensive but seems to work very well for the only thing I use it for (adjusting the mixture).

JYE Tech 06203P 062 LCD oscilloscope: Oscilloscope Probes: Amazon.com: Industrial & Scientific

DISCLAIMER: I'm not an electrical engineer, nor have I played one on TV. So, my observations are certainly subject to review and confirmation or rejection by people who actually know what they're talking about :).

This post should be used as a reference for future problematic issues with a CIS
setup as it provides a good graphical depiction of what occurs electronically:

1. With the oil cap removed, the duty cycle is actually increased, i.e. the ECU
is grounding the FV for a longer period of time to richen the mixture. This
is the same result when one grounds the O2 sensor, i.e. the duty cycle
goes to it's max value attempting to richen the mixture.

2. When the ECU grounds the FV for shorter and shorter times, the duty cycle
is decreased. In the limit when the ECU no longer grounds the FV, the duty
cycle is zero which is the full lean condition as controlled by the ECU.

The term duty cycle typically defines the ratio of an 'on' time (actuation of the FV)
to the total period of the waveform.

Great feedback one and all. Thank You.

I am going to attempt to re-map my digital wurs control pressure and warm up rate first, and then do some re-testing.

In the meantime, all of the talk about the closed loop readings and the factors affecting it, has got me re-assessing my 02 sensor connector, the lead running back to the harness, and any shielding issues I might have.

I have attached a couple of photos of the "current" situation (apologies). One photo shows the state of affairs with the connector and harness lead before I attempted to address it. The harness lead (damaged area) was being chaffed over a long period of time due to it's contact with another harness. It looks to me that the outer cover and the mesh shielding under it has been compromised, but the wires under them are still insulated and intact. I was thinking of trimming away the rough stuff and wrapping the area with a shield of some sort ?
The connector itself had only a few strands of copper wire left attached (via solder) to the connector pin, which was exposed and connected to the sensor black wire socket (I pulled it off for the photo).

The other picture shows what I had hoped would be at least a connector repair ?

I broke away more of the (by now pretty brittle) connector housing/mounting cover to expose more of the green wire and its copper wire. Since it was still difficult to work in that space, I butt spliced (crimped) a small piece of similar gauge speaker wire to the green wire and shrink wrapped it. I then soldered the other end to the original connector pin, shrink wrapped and electrical taped it. This is where the sensor lead attaches now (until I am sure it works).

I have read discussions of the importance of proper shielding and also some about "breathing" sensor leads, so my confidence in the connector fix as well as the connectors harness lead damage, has me a bit doubtful about whether the sensors signal is actually making it back to the ECU accurately.

http://forums.pelicanparts.com/uploa...1413765438.jpg
http://forums.pelicanparts.com/uploa...1413765495.jpg

A common error typically occurs when attempting to repair an old O2 sensor wire.
In repairing the O2 sensor wire, the shield wire gets connected to the center signal
wire which cases the Lambda ECU to operate in its full rich mode. The shield must
only connect to the ground pin at the ECU and remain unconnected at the O2
connector end.

Dave,

How specifically, does this relate to the repair shown ?

Thank You.

Bill

Bill,

There is a lister on this site that makes a kit to repair the O2 sensor connector. Take a look in the archives. There is no breathing going on.

Just do a simple resistance test between pins 2 & 5 of the Lambda ECU with it disconnected.
You should measure an open. Then measure between pin 2 and the O2 connection
in the engine compartment. There you should measure continuity.

Thanks again for all of the great feedback.

Paul, to answer your question, after a 10 minute drive the idle dithers. Maybe 25 rpm or so, up and down at a roughly 1 second frequency....

In your opinion, am I wrong in assuming that the single wire narrow band 02 sensors' black wire (from the sensor to the connector pin) is unshielded ? And, if there is a shielding problem, it is between the pin connection and its path to the ECU ?

I am trying to understand why with a low warm CP (around 2.95 bar) and a .85 V 02 sensor signal (both indicating a rich mixture), the measured duty cycle of the FV is over 80% which as I understand it indicates the ECU trying to react to a LEAN mixture ? One thought I had was that the high 02 voltage was somehow being attenuated on its path from the connector to the ECU, and telling it to further richen the mixture ?

If I checked the connection at the ECU at the appropriate pin (number ?), should I be getting that same .85 V ? If not, and it is lower than .5 V, this would explain the inconsistency right ? I could then focus on that critical path.

Thanks as always,
Bill

Bill,

If the idle speed rhythmically oscillates after the engine is fully warm, it sounds like it is going into closed loop. The oscillation is caused by the dithering duty cycle around stoich. If you are still not reading a dithering duty cycle, you need to try a different meter, then test the green/white wire circuit in the test connector. I would not adjust the mixture until you can read the duty cycle correctly. The duty cycle can be calculated by multiplying the dwell reading on the 4 cylinder scale by 90%. An analog dwell meter displays the numbers normally, not inverted, just as described by Probst and in the Bentley manual, and confirmed by many listers on this site.

If someone could post a model of analog dwell meter that they have gotten to work on a 911 and where to buy it, I think that would be most beneficial (I did a search on our host's site and didn't see one).

I know that all dwell meters are not created equal, and in my case the "Actron" brand meter I have doesn't work properly - reads waaaay too low (and this is the one you will probably find if you walk into any autozone or probably many other FLAPS). I've also read (somewhere here) that you need to use a "powered" meter to get accurate results, but my 60's Sears model that is battery powered yielded similar results. Interestingly, both work perfectly fine on my CIS-equipped '84 VW GTI.

I guess it *could* be the test connector on my car, but the oscilloscope nails it through cold start enrichment, open and closed loop operation on the same wire.

here's an example of one that I know works well (if undamaged)

Sears Inductive Engine Analyzer 28 21423 Testes Volts RPM Dwell Amps Ohms Points | eBay

Look for one with that measures battery voltage with red and black leads and has a 4 cylinder scale