I think what happens is:
- ECU “does not know” if any one cylinder is NOT burning or burning extrelemy lean: it has only one sensor to monitor 3 cylinders
- cylinder pumps as much air as it is supposed to, it is not getting any (or low) fuel
- A/F ratio is lean for obvious arithmetic here, and obviously it is a reason why it is always 0.30V: because it pumps one cylinder of air with no or low fuel
- any leaks on exhaust would show “asymmetric” response to RPM change, but I do not see it, so I have a “leak”, which is proportional to RPMs
- to detect misfire: it has to be either detonation present (no such thing for no fuel) or should be no spark (which is not the case either by my testing)