Mine goes through cycles, where it's lukewarm, to cold to absolutely blistering hot (hot in a way that it should be when on it's hottest setting) - all while never once changing the temp setting on the auto hvac controls. So I'm pretty sure it's not the blend door.

The coolant temp gauge stays perfect.

If the waterpump was going I would think my gauge would be going up or something. But it stays perfect once warmed up.

I don't think a backflush will help you either unless someone can chime in and tell us the inner workings of the heater core. Are there internal fins that get cruded up but flow through the core can still be good? (coolant unable to get through the fins...)

I have great flow through the core, doing a regular or back flush on just the core.


edit: some other details I forgot to add...

On the way home I turned on the rear heater controls since I never really checked what those vents do while I'm having this problem.

Even when my dash vents blow really hot air for a few moments, the rear vents are always blowing cold.

It seems like if I turn the heater completely off in the front and then turn it on again moments later it'll blow some warm air out of the dash vents for a very short period. I have the auto HVAC controls too.