PSALT

This is from the CIS primer:

If the mixture is spot on (14.7:1 air-to-fuel), the duty cycle to the frequency valve is 50%. Note that the default mode for a disconnected O2 and warm engine is also 50% duty cycle.

Please reference your source for the 85%duty cycle. I need to understand the basis for your unilateral statement that no 911 has a 50% duty cycle.

The factory manual, also p 240-26 in Bentley. CIS lambda is quite complex, very different from Basic and most of the information on the internet is a gross simplification or incorrect. What you have copied is incorrect , the duty cycle is never 50% at a 14.7:1 AFR. Have you ever actually seen the duty cycle output on an 80 SC ? The system starts in open loop and the mixture is held rich until the main temp switch triggers closed loop. Then the ECU dithers the mixture back and forth over stoich to get an average 14.7. A constant voltmeter reading off the O2 sensor is meaningless. The system is capable of an average 14.7:1 at idle over a wide range, but never at a constant 50% duty cycle. Th early version of 911SC used a very rich open loop default setting that was an issue for the emissions test. The second version uses a leaner open loop setting, but has to compensate with another ECU, throttle switch and temp sensor to band aid the drivability. The route to understanding CIS lambda is first to understand the sensors, the wiring pin out and the ECU, then watching the duty cycle as it goes through the warmup, closed loop and cycling through the throttle switches. Unfortunately there is no complete description of the system and the test routine online or otherwise.

PSALT - regardless of the default duty cycle, if the O2 system is not connected how can the dwell meter test be effective? I am not posting this to argue so please don't assume that

Bob,


Plug in the dwell meter, start the car. If the duty cycle is not a constant 85%, you know the system is not working correctly in two seconds. It will never run correctly if the default duty cycle is incorrect. Then you follow the test procedure to identify the fault. When the ECU is working correctly, you then observe the duty cycle as it goes through warmup, you can test the operation of the temperature and throttle switches and get a proxy of the actual running mixture once it goes closed loop. There are other ways to test the components, but if the engine runs, the dwell test is the fastest way to get a handle on what is broken.

Af/r
 

DB Gauges: Air Fuel Ratio Guage

flat6 - thank you. Clock hole?

PSALT - That is some pretty heavy stuff. I have only seen the duty cycle in operation using the dwell meter test on my own car - 81. Let me look at Probst's book and send you back a volley tomorrow. I don't have Bentley but have the factory manuals.

Bob, It took me awhile to even figure out where the O2 went. I found the wire from the O2 had been cut. Plus on top of that, it looked like the redneck that was working on it before me, pulled or dropped the engine without unplugging the O2, because the plus was ripped apart. I went to my 82 SC, and that is when I realized what that was. The bad part, is the 80 and 82 have different parts, and I was hoping to swap parts out, but one has a micro switch, the other does not. The warm up reg, on my 77 targa, and the 82 are both vac type, and the 80 is not. Go figure. I have the O2 hooked up now. When I first start the car, it will shoot up to 2k, and then drop right away down to 900. The cold start system is not working at all, and once I got the car to start this morning, I had to let it sit and idle until it was warm, or it would start to pop and die. Once it warmed up, it Idles great, and I could rev it up good, but once under a load, and even before it was totally warmed up you could not get it to rev up without a pop or two, and sometimes it would die.
Before I do anything else, I need to get the filter changed, it looks like the original one, and this car has some miles and the second engine in it.

popping is due to it being too lean. hey! redneck? wait a minute. you can only say that if you are from the south and you are one. just kidding.

the 82 WUR has the vacuum port like the 77?

PSALT - I think we have terminology issues here.

1. Fixed open loop pre-warmup is rich (e.g., the 85%) My generic Bosch reference (Probst) state 60% but I read your comment about Porsche over enriching initially. Cool.

2. Closed loop dithers over the 14.7 AF ratio. Cool.

3. Open loop post-warmup is a fixed 50% duty cycle. If the thermo switch says "warm" the box needs a signal from the O2 sensor to complete the open-loop to closed loop change over. If there is no O2 signal the fixed 50% duty cycle remains and no dithering occurs.

Now to the 50% duty cycle at 14.7 statement made by the author of the CIS Primer. In proper closed loop operation, the duty cycle will never be fixed at 50%. I agree with that. I believe he is making a statement based on theory - but the car is in the real world. Let's end that discussion.

If the O2 sensor is disconnected and closed loop can't occur the fixed duty cycle will start at the fixed and richer 85% duty cycle because the car is cold. Once warm it shifts to the 50% fixed duty cycle that I am calling the "default mode"

If your car is warm - but the O2 is cold and connected - the car will start in the fixed open-loop 50% duty cycle mode and run that way till sensor warm up.

The default 50% mode is what I was assuming Pman912 would be seeing if he used the dwell meter test since he said in Post #1 the O2 sensor was disconnected.

So after all that fun I am standing by this statement; "Every single 911 (CIS w/Lambda) has a 50% duty cycle".

Wur........
 

Porscheman,

Could you check the number on your SC's WUR? Are you sure you have the correct WUR? SC's with OXS use WUR with no vacuum port on the side. Thanks.

Tony

Good point - he did say he put a used one in. If he could get the number a pressure range comparison could be made.

Fuel pressure gauge.....
 

Bob,

I don't think he has a fuel pressure gauge kit. And the absence of this tool really makes the troubleshooting more challenging. I'm giving Porscheman a very high grade for his effort in helping his buddy. Did he ever confirmed that the FV is working?

Tony

Bob,

I'm glad you are interested in what actually happens, because the theory you have been quoting is a generalization and the numbers have nothing to do the 911SC calibration. My advice would be to drive a lambda SC with a dwell meter in the car a few times and you will see what happens and when. Make sure you exceed the 35% throttle switch several times and you will see what happens to the hot open loop duty cycle without assumptions. If you get a constant 50% duty cycle, my advice is to replace the ECU.

There is no fixed relationship between AFR and duty cycle, especially open loop warmup. In closed loop, you can be at 14.7:1 idle with a dwell dithering over 30, over 50, or over 60 duty cycle. The duty cycle is a correction factor and you can only infer what the mixture is by the way the system is trying to correct it. In open loop, the two defaults 85% and 65%, represent a richer than stoich mixture. In closed loop, a needle dithering over 65% means the opposite, the mixture is too lean and the system is adjusting it rich by bleeding off more lower chamber pressure. When you manually adjust the mixture on a lambda SC, you are only really effecting the open loop mixture within a wide range. No engine management system can hold a steady 14.7:1, from the 70's they are all based on the concept of a target and a trim to dither the mixture back and forth over stoich to try and get close to average for the converter life. CIS lambda works the same way and is never near stoich, open loop, hot or cold. When it goes open loop hot, the mixture is always richer than stoich, closer to 13:1 AFR. Once you understand what the system is trying to do and how the switches work, you need a scope to see how the FV pulses trim the mixture with the amount of change and the duration.

CIS lambda is a bit of a bodge and almost like the compound turbo piston aircraft. Once the turbine was making more power, they threw away the piston engine parts.....in this case, they threw away the CIS parts and kept the pulsed injector (FV) and ECU.

Ok, replaced the fuel filter this more, unplugged the O2, it took 4 times before the cars started. After warmup, I checked and reset the timing. That helped, but then the engine started pulsing, and my meter, set on 2v, was shooting over limit, and then it would go way down in - numbers.
I ran numbers on 82, it has an 81 engine, but have vac port on warm up reg. My 77 is a 2.7, and it has the same as the 82, or 81 engine. The place the vac port would be in the throttle body, is blank.
Now it seems to be running good, other than with the O2 disconnected, it feels like what a connected bad O2 does, maybe that is bad. When I rev it, it will pop once it gets up into the revs. I amn about finished with this. I hate to give up, but I have paying jobs that I am not doing, because I am trying to get this done so the girl that owns it can drive it to Home Coming.

Sounds like the mixture is off, put a dwell meter on the 4 cyl scale on the test connector and it will tell you what to do next. If it does not fire immediately cold, you need to diagnose the separate Cold Start Valve. This fires the engine, then vacuum lifts the sensor plate and hopefully supplies a rich enough mixture to keep it going. If it fires OK, but immediately stalls cold, the mixture is too lean, maybe for several reasons. Most lambda SC's cold start better with the vacuum retard (grey hose at the back of the dist) disconnected and plugged. Old engines with vacuum leaks and lower compression can't generate enough vacuum at cold start and they have a much stronger idle without the retard. You will need to reset the idle speed to 950 rpm with your fingers on the big bypass screw on the throttle body .

ok, I need to check one more thing, the dist has to vac hoses, one blue, one red. The red one is plugged into the back, or front of car direction, of the throttle body, and connects to the vac side of dist, side away from dist. The blue one, hooks to the side of the throttle body I can see, and is hooked to the dist side of vac can, is the right?

which way fattens this up, clockwise or counterclock wise?

Paul - I am with you. None of my discussion entertains anything other than generic operation.

Tony - Nothing on frequency valve yet that I see. Was thinking on the WUR we could find graphs that would indicate pressure differences that would hamper his car even if everything was working ok.

Pman - walking to garage now and will check hoses and report back.

ok, I need to check one more thing, the dist has to vac hoses, one blue, one red. The red one is plugged into the back, or front of car direction, of the throttle body, and connects to the vac side of dist, side away from dist. The blue one, hooks to the side of the throttle body I can see, and is hooked to the dist side of vac can, is the right

The red one is the advance and goes on the outside of the cannister, pointing toward the rear of the car. The grey or blue one is the retard goes on the other side of the cannister. Sometimes they are mixed up, the retard uses manifold vacuum, full vacuum at idle, the advance uses ported vacumm no vacuum at idle. A timing light will show the effect. Disconnect and plug both, set the idle to 950 and then the timing to 5 BTDC. Reconnect the red and leave the blue plugged.

Clockwise richens the mixture, but if you have an operating lamdba system, do it with the dwell meter connected and you will get the best results.

Distributor and test port

http://forums.pelicanparts.com/uploa...1286304056.jpg
http://forums.pelicanparts.com/uploa...1286304088.jpg

Ok, boy did I find the problem, and my figures are still hurt. That CDI unit bites hard, let me tell you. I started hearing a faint ticking sound, which is amazing I even heard it do to bad hearing from machine work, fab, and Artty when I was in the Corps. After the CDI decided I needed to be bit, I moved the plug wires a little, with car off. I started it back up, and what do you know, it is running smooth. I thought I was about to burn it to the ground this morning. Thank you all, and Psalt, thank you for the hose info, they are right, but I just needed to check.