i have a 99 gmc suburban k15004wd that has no passenger compartment heat. The airflow director works great. air blows on floor/defrost/vents with no problem. The problem I have is that it never gets HOT. The A/C works great. I just need heat. Is there a seperate temp control "door" or acttuator that may be stuck? The heat doesn't wrk in the back either, but they "broke" about 8 months apart. front heat worked fine for a while, but now i get nothing but cold air. Heater hoses seem to indicate water flow all the way to firewall as they get hot, so i don't think its a water flow issue... but..

Any ideas? I came out late from work last night and my 5 year old was snuggled up to his mom because they were sol cold waiting on me.! Help!

 

Welcome to the forum.

The temp. blend door/actuator may be stuck.

 

I suspect this is the issue. Any idea how to get to it? I'm somewhat handy, if I just know what to do. Know where I could get a diagram or basic "how to" to check it?

 

oh, and thanks for the reply.

 

okay, I got to tinkering, and i've determined that the air flow IS different when i turn it to cold vs turn it to hot - the air flow is "slightly" reduced- likely because it's pushing thru the heater core. SO - PERhaps I'm back to a fluid flow problem. Under the hood there is a "valve" that is vacuum activated. I noticed that the vacuum pressure going into this (coming from the little hose) is quite small. It connects to this little "valve" thing right behind the A/c (dryer?) on the firewall. There are 2 vacuum lines and one electrical port. It's about 2 inches tall and 1/2 wide? The vacuum going INTO the device (coming from the engine) is quite strong - but the vacuum going into the valve is reallly weak. Do you know what this valve is called? Do they go bad? I've replaced the heater control valve that the cooling system heater hoses connect to about 6 months ago, so i think i may indeed have a vacuum problem. hose doesn't look leaky. Any ideas what this "vacuum control" valve is called?

 

Take those two vacuum hoses off that switch and connect them together directly.
You might be able to use the empty portion of an ink pen ink tube to butt them up.
See what happens then.
Maybe the switch is bad or the switch is not getting 12V from the dash controls.

 

Okay, I did as you suggested and connected the two vacuum hoses to eliminte the switch. When bypassed, it didn't change a thing. I have verified that the heat valve is opening and closing correctly but to be honest - this "seems" like a water flow problem and I'm not real sure how to diagnose from here further. Here are my findings:
I can feel heat in the hoses up near the engine block but the further i get to the "valve" it's cooler and cooler. On the other side of the valve it's just plain cold. I've disconnected both hoses at the first connection point off the engine water does seem to be flowing thru them but it doesn't look like there is any circulation past the firewall. I'm basically convinced that the reason there is no heat in the front or the back is that there is no water circulation. But for the life of me i can't figure out if there is supposed to be a high pressure side or low pressure side or what... There's the end that comes off the water pump and then the end that comes off the engine near the thermostat. The one near the thermostat has that quick release thingamajiggy (which i had to replace today as well). I ASSUME the hose coming off the water pump is the "sending hose" but it sure doesn't seem to be sending much. I replaced all the hoses a while back - new radiator, water pump, etc. Perhaps I reconnected them incorrectly. Any one by chance have a hose diagram. From water pump, it goes into the "Y" joint, one end of the "Y" goes to the firewall, the other one goes into the heat valve.

Thoughts, ideas, comments solicited - PLEASE

 

okay , i got a cooling system diagram from the dealer, and learned that the hose off the engine block is the sending hose... it's either got some sort of an air bubble in it or the valve is bad internally - it LOOKS like it's working but it simply can't be. I'm going to replace it.

 

I was going to suggest that you replace the valve w/ a piece of tubing cuz it sounds like the valve isn't opening.

 

Isn't that the same valve you replaced six months ago?