I bought my`90 Blazer new.It was my wifes driver/family car.It never seemed to have real good heat.The more time that passed,it got worse.At first,only my wife complained and I dismissed it.It worked better with less fan,I`d tell her.The coolant always looked rusty and I tryed flushing it many times.I went to Jiffy Lube and they flushed it for over an hour and it still wasn`t totally clear.I had to replace the radiator and heater core at an early time.I`ve replaced the radiator once since.It now has 180,000+ miles and "NO" heat.I drove it a little last winter and hypothermia was a concernI`m dreading winter coming.
What the heck could this be?Whatever the rusty water thing was has stabilized at some point.I wonder if my heater core is just plugged up,just not leaking.I guess I should replace that.Has anyone seen this before?I never have.

Both heater hoses are warm.I think the core is gunked up and blocking all but a tiny area.

I agree about the heater core . At 17 years old it will start leaking so nothing lost if you

. replace it .

I usually undo both heater hoses and put the water hose to 1 of the to flush the gunk out, the flush from the other direction. If the heater core is not plugged you could have a door sticking due to a vacuum problem or a stuck door hinge.

Are you refering to temp control or directional flaps?I don`t know anything about how these systems work.The flaps that direct the air(heat/defrost)work.I know you mean temp,I guess,because there`d be heat from one or the other if that was stuck.My mechanic never suggested this and I wondered the same thing.The levers act normal and move freely.And,I can hear the vacuum sound.

I would try the flush first since the vacuum and doors and such can be a royal PITA sometimes. The under-dash area is definately the most aggrivating part of these trucks.

take both hoses off and stick an air hose in one of the tubes. that is how i cleaned mine out.

Leave the radiator cap off and look at the return coolant flow with the truck running at operating temperature, poor return flow would suggest a bad t-stat, plugged heater core, or worn water pump. I have normally found that when the pump is bad, the heater gets cold at idle and gets hot when moving.

It`s been this way for a long time.I doubt it`s the pump.I`m going with replacing the core.Yesterday when it was pretty warm but raining,I used the defroster and cranked the temp lever up.It had a trace of heat,came out with the heat switched on`too.
I absolutely "HATE" doing anything under a dash.It`s downright painful,to say the least.

if both hoses are warm, wouldn't that mean coolant is flowing through the heater core? You may have a problem with the vacuum switch on the back of the control head on the dash. There is a (usually) 9 port switch that moves vacuum to all of the doors. The levers will work freely, but just lose function. The usual first sign of this is everything blowing from the floor vent, and slowly moving to the desired position.