High dwell on 911 SC ‘81 lambda (US)

[mysocal911]

Originally Posted by Funracer

Is everything working well enough to set the mixture to 45%?

Thanks

Hopefully, you are aware that, since the O2 sensor lacks a heater, the engine must be warm and the RPM should be about
1500-2000 to set the mixture. Once set, the DC should vary slightly around 45%, and the O2 voltage will vary about +/- .20 volts.

[Funracer]

Originally Posted by mysocal911

Hopefully, you are aware that, since the O2 sensor lacks a heater, the engine must be warm and the RPM should be about
1500-2000 to set the mixture. Once set, the DC should vary slightly around 45%, and the O2 voltage will vary about +/- .20 volts.

Why does the rpm have to be 1500-2000 instead of around 1000 (idle)? Is that just to make sure the O2 sensor is warm enough to generate voltage?

Thanks

[mysocal911]

Originally Posted by Funracer

Why does the rpm have to be 1500-2000 instead of around 1000 (idle)? Is that just to make sure the O2 sensor is warm enough to generate voltage?

Thanks

That's correct! Once fully warm, e.g. after driving for 30-40 minutes, it'll provide a good signal at idle.
For some O2 sensors, though, that might not be the case. That problem was eliminated on the later
Porsches by adding a heater.

[AndrewCologne]

Originally Posted by mysocal911

Once set, the DC should vary slightly around 45%, and the O2 voltage will vary about +/- .20 volts.

I figured out that if the base CO setting is set at 0.4-0.8% (factory default), then the idle duty cycle on a warm engine jumps around 60%.
And thats normal as the 0.4-0.8% are leaner than lambda 1 and after the ECU regulates the mixture back to stoich/Lambda1 a typical CO of approx. 1.5% is the result (before cat!).

Many people (like myself) do suggest setting the CO screw so a 45% duty cycle at warm idle. But this also sets the cold start and WOT enrichment out of spec and in fall/winter in cold regions like the NE or in Germany the cold start for example will result far to rich.

[Funracer]

Finally, after at least three months trying, I was able to accurately measure and set my FV today to a duty cycle of 45% and measure my O2 sensor output at .52 VDC at the same time. I was never able to do it with a DVM. AndrewCologne, Dave at mysocal911 and the guy at Systems Consulting whose name I cannot recall (sorry!) patiently lead me through the long process until it all came together. I believe it was Dave who mentioned that the FV itself can sometimes interfere with the DVM reading and this ultimately was the key to getting it done.

It’s really not hard to do with an oscilloscope, but who has one of those?? I do. $42 bought a cheap one on Amazon (see pics above), and surprisingly it worked great.

Just to make it easier for anyone looking at this in the future, I’m going to submit a separate post on how to use the scope. I was a bit intimidated by the thing but it really is a cool instrument. Now that I have an accurate, repeatable way to measure my mixture I can move forward with other projects.

And finally, an apology to wosm for sort of hijacking his thread.

Thanks again everyone!

[wosm]

@Funracer, great you solved your issue! Beside Dave and Andrew, you also contributed to this topic , so lots of thanks too

I had some busy days at work and was not able to drive my 911 sc until today. I measured pins 7 and 12 both during idling and the Voltmeter showed me ~12V. After driving for more than half an hour I used my oscilloscope again. Results on dutycycle were exactly the same with engine running (50.6 % with pin 2 disconnected; 35.7% with pin 2 disconnected and pin 7 grounded; 26% pin 12 grounded to pin 16). My Co is 0.8 without cat. So I think my ECU functions also OK. However, it is still very strange that nothing changes with idling when I remove my relais under the passenger seat or ground pin 2. It stays one big puzzle for me

I still don’t know what is causing my engine runs that bad when cold and hesitates around 2000 rpm. No vaccum leaks, ignition timing re-set according specs, fuel pressures OK, etc. When the engine is warm the hesitation/stutter is less at 2000. The car doesnt hesitate when I give more throttle. I think I need to inspect my plugs or use others to test any difference. Also I have some doubts on algae in my tank and/or fuelfilter causing this misery.

[AndrewCologne]

(50.6 % with pin 2 disconnected; 35.7% with pin 2 disconnected and pin 7 grounded; 26% pin 12 grounded to pin 16).

Would mean you connected your DMM the wrong way as 35% should be 65%, means inverse. This will be achieved by a proper inverse conneotion of the DMM's terminals when reading duty cycles where the ground signal gets interrupted by the duty cycles pulse modulation.