Lambda CIS: US CO Spec vs. Closed-loop duty cycle

[schoward]

Splitting hairs a bit I know but....

I recently went back to running my O2 sensor/stock CO specs on my US 81 SC (vs. running rich with no O2). Car runs surprisingly well with great driveability after a fresh O2 sensor, fresh O2 relay and a new male O2 connector from Louie85.

I set the mixture using my Gunson to 0.6-0.8% at the cat test port (O2 unplugged). Took multiple readings on multiple days and all were same result. Plug O2 back in and CO goes to 1.0 -1.2 at cat test port.

I then put a good dwell meter on test port in engine bay and observed a consistent closed-loop dithering between 49-54 on the 4cyl scale (55-60% range duty cycle). Bosch book says target of 45-55% duty cycle (50%) indicating I am slightly lean currently.

I would like to set to the proper factory spec, see how it runs and then go from there. I also have to pass emissions soon so want to start out at baseline specs. I know Gunsons can have a decent margin of error, so question is this:

Is the US CO idle spec slightly lean by design and the O2 corrects it (my Gunson got CO spec pretty much right), or should the US CO spec of 0.6 +- .2 before cat = ~50% closed-loop duty cycle which means I am still slightly lean against the spec (my Gunson was a little off)?

I have to believe based on dwell readings I am probably very close to hitting the US spec settings if not there already (depending on answer to question above).

Thanks in advance.

[Peter Zimmermann]

1981 CIS adjustment procedure:
1. Engine at running temp - front oil cooler hot to touch. Idle speed 900 +/- 50 rpm.
2. Oxy sensor disconnected.
3. Measure exhaust in front of cat.
4. CO% spec = 0.4-0.8
5. Low side of spec is acceptable to prevent possible idle fluctuations.

[schoward]

Peter, Thanks for the response.

Yes - Think I did pretty much that procedure using my Gunson. CO in range 0.6-0.8 at cat test port; O2 unplugged ; at operating temp and in-range idle.

But Dwell on ECU test port indicates slightly lean setting in closed-loop operation. Reading of dithering 55-60% closed-loop duty cycle vs. around 50% per the Bosch repair book which has me questioning.

At proper CO spec, I would have expected dithering around 50% duty cycle unless my Gunson wrong, OR that's what the stock CO idle spec yields by design - slightly lean and O2 corrects it (perhaps for emissions?)..

Disregard the dwell reading? Disregard the Gunson?

Thanks.

[matt demaria]

If tou read the instructions for the Gunson tester, it is accurate to + / - 1% CO. It is not accurate or precise enough to do this.