If this helps at all, it's on my '91 300SE with the 103 engine, 49-state.

Key on engine off, but warm:
- DC = 50%, always has since 1991.
- Open the throttle between 10% and 90% open, DC = 10%.
- Open the throttle all the way and engage the under-pedal switch,
DC = 20%
- Press the air valve down, DC = 40%.

Crank the engine:
- DC switches momentarily from 50% to 70%, then back to 50%.
- After a few seconds, DC begins to fluctuate around 50%, as O2 sensor
comes in.

All the above makes sense to me:
Initial 50% without lambda control is normal, either without engine running, or before O2 sensor warms up.
Opening throttle with engine off should set a 10% fault code, which is throttle position fault.
Opening the air valve with the engine off should set a 40% fault code, which is an air valve fault.

So, I conclude those portions of my system are performing OK.
But I still have the 5-second hot-start chugging problem, which I have been chasing for 10 years.

DG
o