this is long!!
I have a 318i with heater not heating well its either really cold
or really hot.
I had read that changing the thermostat will help,
i did this and highly highly recomend it.
Now heater works! BUT it has climate control, 19 cold 21.5 cold
when i change it to 23 its warm and 25 hot, the hotwater
changes on the outlet line behind the dash after having it
all out to test!
So hot water in and out ok OK!
But there is also 1 flap with changes the temp and seems
delayed sometimes works ok sometimes not!!
Does bwm scanner help with this! fault codes etc