1996 Z3 1.9L: heater not blowing hot

[Gerard C]

Engine was not warming up. Gauge was staying in the cold blue zone. I changed the thermostat and put in new coolant to the proper mixtures and bled out any air. Engine is now warming up to temperature and gauge is pointed directly in the middle/up where it should be.
At idle heater only blows close to the ambient outside temperature, about 80 to 85°. It only blows hot when the engine is consistently revving a 2 to 3000 RPMs. Only one hose at the heater core gets hot. I’m guessing the inflow to the core (see white arrow).
The other hose does not get hot. iIm guessing that’s the outflow from the heater core then goes into some sort of electronic temperature sensor/solenoid/valve (white circle).
Not to sway anyone’s diagnosis but I’m thinking it can only be partially clogged heat core or that temperature sensor.
Thank you in advance for any and all input.