Just bought a 98 Blazer 4wd 4 dr. Love it- but one issue:

The heater. it works a little too well... The heater is like "wow!- woa!!- umm!?-- ow!... OW!!!! turn it off TURN IT OFF!!"

If a walk-in sauna is typically 140 degrees F +, then this heat coming through the vents is 200+. Its literally scalding; even when HVAC is turned off, the natural venting through the foot vents scalds my toes on the gas pedal.

I am worried of a meltdown/fire- and the heat on long road trips causes my stereo to cut-out from overheating it is so bad (good?). I mean, maybe its a great thing for Canada, but I am in SoCal.

The blend door actuator was found to have cracked gears, and was replaced, the blend door itself moved freely by hand from stop to stop. The air temp control (manual) was read to read varying voltages across entire band, the replacement actuator was seen to drive full range. The blower motor functions properly at all settings, the vacuum actuated diverter actuator functions properly as well.

I have a working Air conditioner- that works fine- until the engine warms up, then the heat overpowers it- and the vents blow hot air.

So... what could be the problem? blend door has hole in it? Heater core just too awesome?

TL;DR: my heater is too strong. always blows hot air- temps range from fully hot to OMG HAWT!!

 

Very first step is connect a scan tool and see what the coolant temperature is when the engine is warmed up.

 

just bought a scanner.

Coolant temp rose to 189 F and stayed there. On freeway and at idle.

Additional info: when set to "natural vent" position, or to OFF- full heat still comes in.

If its not the temp control door- is there another "diverter" door in there somewhere that may be stuck/faulty?

 

Actually that is slightly low. The thermostat is designed to begin opening at 195F, and regulate between 195F and 200F. Both heater hoses should be nearly equal temperature, somewhere in the 160F range at their connections to the heater core. Register outlet temperature is typically ~130F.

 

Both of the hoses leading to the firewall get extremely hot (too hot to hold onto).

I had a (nil experience) guess that maybe the heater core was the only acting radiator for the engine somehow... but a neighbor assured me that wasn't possible.

I'm going to get my laser heat sensor to measure the heater core hoses, as well as vent outlet temps.

 

If the ECT sensor shows coolant is regulating at 189F, the heater core, and heater hoses, will not exceed that. Infra red thermometer will tell the story. If the cooling system is in good condition, these things put out amazing heat.

 

so then its simply a matter of "closing the heat off" within the HVAC module

.. guess theres a leaky door letting that a-mazing heat in- no matter what?

As stated in OP, the blend door actuator was replaced, and the door axle itself was hand turnable to both stops (no obstructions). The new actuator was seen to drive fully in hand before install.

Maybe its the door itself...? (maybe its gaskets should mean its NOT turnable by hand?) I've been trying to get google images of its internal workings, but am unable to get anything relevant.

 

This might help. Across the bottom, (right to left) it shows air flow through the heater case. It's a huge image, but it it's clear:

 

wow thank you so much for that!

So I've repaired replaced the "air mix door actuator"- but there are several pneumatic actuators to check out for the actual "shunting away the heat source" mechanism

 

The diagram above is a "typical" drawing for illustration purposes only. It is not exactly how your system looks, or operates, but it's close


The "air mix door", commonly referred to as the "temperature blend door", is controlled by the black plastic electro-mechanical module.


The only door that affects heat, other than the blend door, is the "Slave door". In its default position, the slave door forces all air flow through the heater core. This allows you to adjust the temperature when the mode switch is in the OFF position, and or the system loses vacuum. If the slave door is stuck in the AC position, (like in the diagram) no air flow will pass through the heater core, and you will have no heat at all. That's probably NOT the case with your vehicle.