I have a 91 Silverado that will not put out hot air. It will blow when you have it on the Heat setting but it will not provide hot air. What could be the problem? Does the Heater Core have some type of valve on it? Thanks for any help that you can provide.

usually no valve for heater core unless your a suburban with rear heat check to make sure coolant is full and thermostat is working (hoses to core should feel hot). if they are you need to make sure doors under dash are working. when you change your temp setting from cold to hot usually there is a noticable diffrence in flow. any other questions just ask.

Check the temp actuator under the dash, they're known for going keplunk. Been awhile since I've done one on a GMT800, can't remember which one it is.

Before we get real technical... how's the thermostat? Is it full of water?

No heater valve on GMT 800's, unless someone put one in line.

Warm it up, grab heater hoses and see if they're hot... only saw it once but the passage out of the intake on one particular truck was blocked

Thanks, I will check these items out and post my findings. Its nice to have a place to ask questions like this and get solid feedback. :cool2:

Well, the hoses in and out of the Heater Core are hot, so it appears that the little door in the air box is not moving. It there any place to get informtoin on how to get into this part of the dash and make repairs? May just have to take it to a Professional rather than do it myself. Thanks for any input you might have.
:)

put the blower on high and then switch from cold to hot see if there is a diffrence in air flow not temp just volume and sound that will tell you if door is moving.

No change in air flow. Looks like a trip to the shop.

No change in air flow. Looks like a trip to the shop.
dont jump the gun yet let me do a little research for ya. do you have a digital volt ohm meter? I can send you some wiring or trouble flow charts.

yes it do have one. That would be great. Thanks for all of your efforts.

I will try to help you out when I get to work on wednesday.

Looks like a temp actuator in someone's future!

Then again them smart boxes (control head) are known to got kaput.

This summer for some reason my a/c started misting water into the cab, like the drain was plugged and the condensation was accumulating in the evaporator core. But it was fine the next day and never had a problem after that, or so I thought.

About a week later I noticed my hot/cold actuator on the drivers side was not working (stuck on full cold). Eventually I took it in and it got fixed under warranty: the actuator had failed.

I wonder if the mist, or some water dripping that might have resulted, toasted the actuator...

MSDawg, I have an extra control head from an 88 or 89 that is black in color and isn't as fast (cold to hot scan) as my 90. Ended up finding a connector ground burned instead of the box. If you would like to have it mailed for testing purposes, let me know through PM.

Thanks Reineke, I am going to check a few things this weekend and if they don't work I may just take you up on your offer. Thanks for everyones input.

It's fixed. Thanks to each one of you.

What did you end up finding?

:exactly: Yeah, really! You gonna just leave us hanging after all that?

Sorry about that. It was a loose connection on the actuator that opens and closes the door to the heater core box. It is still a little stubborn so I may end up having to replace the actuator. Took a little while to find and get to. I had to remove the Glove Box and Tape Deck/Equalizer to get to the actuator. Once these were out it was easy to get to. Thanks again for you help.:)