I have a 99 ZR2. a few months ago i replaced a heater core that was leaking. when i got the dash all back together i tested it. I let the engine heat up a little and turned the heat on. the air coming out was not hot. I thougt well maybe the coolant isnt hot enough to notice yet. I let the engine warm up the normal operating temp. still no heat. Everything else seems to be operating normally. it blows on the floor when i put it on floor. it blows on the widnshield when i switch it to wind shield. The a/c works fine too. Anyone know what could be going on? Im guessing the door just isnt opening on the heater core. I dont think the vacuum line came unhooked because i dont hear any vacuum leak. does a new heater core need to be primed when you put it in? could there just be no coolant getting into it? I dont want to have to take the dash apart again. Can someone give me some ideas?

 

Did you make sure you have purged all the air from the coolant system and that the coolant level is correct?

 

Yeah, air in the cooling system would be my first guess as well. Air can get trapped in the heater core, prohibiting flow of hot coolant through the core. If you are confident in the rest of your work, this would be the first place I would check.

 

Well i felt the hose going in and out of the heater core they were both hot. i took one of the hoses off and coolant came out. I didnt drain the system completely when idid it. i just took the hoses off the heater core and let it drain out the top. Where can i purge the system? I thought that the temp control was operated by vacuum, but i took the controler out today and there is an electronic rheostat or something of the nature behind the temp control. could that be the problem. i dont have my service manual with me so i cant figure out how to test it. i just used a multimeter on the ohm setting. i put the too leads on the middle pin and outside pin and i got a reading when the dial was turned all the way to the cold side. then i put the leads on the middle pin and other outside pin and got a reading all the way on the hot side. i didnt get anything in between in any other combination of touching the pins. i want to make sure that is the problem before i buy a new one. im sure they arent cheap. thanks for the help

 

well i took a look at the door from the engine side of the fire wall and the door isnt moving. i pushed the door over so the air coming in would be going through the heater core and i have heat. now to solve the problem of the door not moving. i dont remember from when i took it apart, is that door operated by vacuum? any ideas on why the door isnt moving. i dont wan to drop the dash again if i dont have to. thats the only actuator you absolutley cant get to through the dash anywhere. any ideas would help, thanks

 

Bringing this back up!! I have a similar issue. I have heat as long as I don't turn the fan up too high. When I do it blows cold. When switching between a/c and heat I do not hear any "clunking" like you would normally hear indicating the door moved. How canI check it in my 99 bravada?

 

sounds like that could be thermostat stuck open. i dont have a clunk when i switch from a/c to heat. the door that controls that is controlled by an electric servo, or motor. it doesnt really move that quickly (enough to hear a clunk). what does the temp guage say when its at operating temp? if the thermostat is stuck open its probably not getting hot enough. when its on high, its enough to cool your heater core down enough that youre not getting heat anymore. my problem was a stupid mistake. i didnt plug the harness into the blender door motor.

 

Temp gauge has always been in the normal range. I get heat until the blower gets turned up. I did a test and had the AC on and turned the fan all the way up, then switched the AC off and the air flow changed dramatically like something was in the way.

 

heater core might be clogged up not allow enough coolant through to keep it hot. if you feel the 2 hoses going into the heater core at operating temp they should both be hot, and not much difference in temp between the two. probably not enough to notice by touch. if they arent both hot you can take them off and flush fresh water from a hose through the heater core. this is not an uncommon problem with dexcool systems, at least with the s series. i had this problem when i firts bought my blazer i had no heat at all. i flushed and fixed it about 4 times before i ended up taking it somewhere to get the whole system flushed.

 

I had this exact same issue after replacing the heater core on my '01.

The blend door is operated by a stepper motor, not vacuum. In my case, leaking coolant somehow got into the electronics for the blend door motor. A new motor was expensive so I used small gauge wire (18 - 22 ga) and soldered across the traces that were corroded. I don't remember what It looks like well enough to tell you exactly where I soldered, I've slept since then(it was over a year ago).I think I soldered from the connector to component posts, soldering wire to traces is dificult ... for me. If you repair it like I did, use hot glue to secure the jumper wires so they don't break loose due to vibration.

I did this because I couldn't exactly make it worse... and I'm a cheap bast***...now the bad news... you'll have to pull the dash out again to get to the stepper motor, if you have the same design as I have.