I'm getting two messages here.

"Measured w.r.t. ground, the scope showed a decrease in duty cycle when the load increased.
The way I see it is that a lower duty cycle causes less force to work against the spring tension inside the IAC leading to a larger airgap."

I see from your scope trace what looks like probe on ECU-33 and ground reference. In that case the duty is opposite what the scope reports because the duty is to open the valve (against the spring) not close it. I think maybe that's what you meant. More power to the valve means more airflow.

Anyway, the point is to understand that the change in idle speed correlates to the ECU-provided air, and not to a leak. If the ECU changes the idle by closing the valve, it is trying to compensate for a leak.

As to the true RMS feature being helpful on pulsed DC, I can't answer that. My true RMS meter is a Fluke which I originally bought to check power from inverter waveforms on the data network equivalent of cable television systems. Long ago. Before Agilent was invented. Have no idea how it affects the meter in the DC mode.

--
Art Benstein near Baltimore

Speaking of Yale "You'd even sometimes hear that we went to HLS (Hogwarts Law School). It's telling that the best way to describe the law school was a reference to a series of fantasy novels." - JD Vance