heat on then off

[bil-loc]

my heat only works while driving. it turns cold while idleing is this a thermostat problem. help.......................

 

[rrubel]

I had that identical problem recently in another vehicle. A vacuum line was disconnected. I don't think the line itself had anything to do with the heater (it actually went to the fuel delivery system), but the lack of vacuum must have kept a sensor from staying open/closed or something similar.
[RICHR]

 

[duals]

Very possible that your antifreeze is low, this happens when the vehicle is idleing at a light,etc. Worth a check.

 

[Top]

Are you getting a 'low coolant' light on your DIC? What is your temp reading at idle v. moving? The engine temp should actually get hotter when sitting still. I'm betting you get 'heat' from moving because of better airflow through the car. I assume your blower is working on all speeds. Given that I think your vacuum controlled doors are not opening. Cycle your selector while sitting still. Slide it thru ac/heat/vent/defrost slowly and listen closely to see if you can hear the various doors open. Check for broken or disconnected lines. Don't assume they are good until you check them. "Looking" connected is not the same thing.

Good Luck

 

[BigRed]

Yank the thermostat and refill the cooling system...properly

If that doesn't fix it ya check out the vaccum lines. When you rev the motor up its drawing more vaccum and releasing the heater door and when its at idle it doesn't have enough juice to keep it open....

Common GM problem but eliminate one problem before the other.

Also this is a good time to see if your AC Evaporator is clogged this will slow down the amount of air coming from your vents.