With the engine off, key out, the heater valve is wide open.

With the engine running, heater off [edit, corrected from “on” as noted in next post], the heater valve is closed.

There’s only a heater valve on the return side of the cabin heater. Even with the valve closed the heater core is subject to cooling system pressure from the supply side. Coolant just isn’t flowing.

If you know which monovalve pin is which, you can remove the connector and supply 12V directly. Then you can pressure test with engine off, key out, with 12V and without 12V to see if there’s a difference in static system pressure test. It’s another datapoint.

Sixto
98 E320s sedan and wagon
02 C320 wagon