New AFR Gauge How to Use It?
 

While the engine was out last year on my 83SC with stock CIS, one of my upgrades was to add a wideband AFR gauge using an AEM controller to help me more easily set my mixture.

I know (I think) that I am shooting for an AFR of 14.7 at 1000 rpm idle, correct? The tricky part is the car now has two O2 sensors, one on each bank. The original one varies the mixture using the frequency valve to maintain the stoichiometric ratio and has no input to the AFR gauge. The new one does not modulate anything, it only displays an AFR value on the gauge.

If I disconnect the original O2 sensor and set the mixture to an AFR value on the new gauge of 14.7 at 1000 rpm, when I plug the original O2 in again it drives the mixture much leaner, like to 15.7-16.0. I would have thought with the mixture already set to 14.7, it would stay there, or very close to there, when the O2 is plugged in again and tries to maintain stoichiometric.

Lots of AFR gauges out there. How is the best way to use it?
Thanks

All I know is that the idle AFR should be around 14,7 as you also expected. My experience on my '81 US SC with stock CIS is also that the idle AFR commutes around this value and it heavily depends on external conditions like wheather and temperature conditions, cold or warm engine, etc. The better the condition of all CIS components is, the better and less sensitive will the CIS be. Every bad components might have a big influence on the mixture!

The leaner the mixture, the hotter the combustion. On idle this won't be an issue. But while cruising it has to be very close 14,7 as the lambda control within the 930/16 engine to perform optimal the catalytic converter. And most important - for optimal combustion, power, acceleration, temperatures and fuel economy with more than 35% throttle or WOT the AFR must be 12,5 on aircooled engines!

Most CIS operated engines seems to run richer than necessary because of inaccuracies of the CIS system evolved over time/wear or false air/air leaks. On the other side, the lambda control on the 930/16 engines is able to clean out these inaccuracies mostly, but this is also limited (not lower than 20% duty cycle on rich mixture, upper limit is 70% duty cycle).
With a second AFR gauge you should see this. But - if you mount the gauges O2/lambda sensor only at one cylinder bank you cannot be sure, that the other bank is working correctly. Mostly only one or two cylinders are disadjusted (worn injectors leads to bad spray patterns or leaking injectors) or the fuel distributor needs an overhaul. But this will ruin the whole mixture control with all bad habits (bad fuel economy, less power, smell for fuel because of uncombusted carbons, surging on idle, bad start, etc.)
Are you sure, that your O2 sensor (the original one for the lambda control unit) is working properly? You can measure it's signal with a digital multimeter, it should commute between 0 and 1 volts on the white signal cable (warm/hot O2 sensor. and engine) When a O2 sensor is worn, the voltage interval will get smaller and smaller. New O2 sensors have bigger intervals. If the voltage interval is smaller than the interval between 0,2-0,7volts, then it mght be damaged and should be replaced for testing purposes and to see how the lambda control unit reacts on this. If the behaviour remains the same as before, then I guess the problem is anywhere elese on the CIS, e.g. fuel distributor and/or the injectors are bad or you have a air leak in the CIS system. Air leaks are the biggest enemy on CIS engines.

Best way to measure and display the AFR is to mount it's O2 sensor mostly close to the original one so that exhausts from both cylinders banks are be measured. Everything else is for debugging purposes not meaningful enough.

Thomas

I might be wrong but I believe when you disconnect the O2 sensor from the lambda controller the frequency valve maintains a 50% duty cycle. So you are adjust the wide band and the AFR gauge with a 50% duty cycle. Reattaching the O2 sensor allows the lambda to control the frequency valve duty cycle when fully warmed up based on the O2 sensor readings.

Again, I might be wrong but wouldn’t you adjust the mixture with both O2 sensors attached, and warmed up. You are using the wide band AFR gauge for the baseline AFR of 14.7 and the lambda is actually controlling the mixture once the baseline is set. Adjusting the fuel mixture for the wide band gauge at 14.7 and 950 rpm’s should also be satisfying the O2 sensor attached to the lambda if the lambda system is functioning normally.

I did not mention this and should have. Another change during my engine out was to install SSI headers. So I have separate exhausts and a two in one out M&K muffler. Should I have put the AFR gauge O2 sensor on the same side as the lambda O2 sensor? Not too hard to do if that will work better.