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).
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).
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
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