No heat: heater valve, clogged heater core, or headgasket?

[Coldaccord]

I have a 97 M3, automatic (it's my daily/winter car) with 213k. I bought it a couple months ago, temperature gauge wouldn't go past ~1/3. Presuming it was a thermostat stuck open, I purchased it, replaced the thermostat, and all was well.

Fast forward a month and a half: I lost heat intermittently for a couple days, then it was gone completely. I bled the cooling system again, issue persists.

Possibility: slight headgasket leak allowing air into the system. I'm not getting any chocolate milk froth in my oil though. And the temp gauge stays where it should be (thought it did spike and return shortly after, perhaps alluding to air pocket). Perhaps it just needs to be driven more for potential headgasket leak to get bad enough to really rear it's head?

But that leaves me wondering; could it be a faulty heater valve, or perhaps debris stuck in the heater core? If so, I need to pull the heater valve, check for crap stuck in it, and bench test it to see if it works properly. At this time I should also blow out the heater core, is compressed air adequate (maybe using one of those nipples to allow it to get a seal)? Unless there's an easier way to check heh?

I often get "check coolant level" warning after a drive cycle more than a few minutes, can't check the coolant level because it's hot obviously, but that again points to headgasket, pressure in the system.

Just curious if there's any input to any other possibilities before I pull the unit and try to clean out the heater core? Or maybe do a headgasket, sigh.

[Braymond141]

Heater valve failure is common enough that I would focus on that.