heater problem

I have a 1990 Chevy Lumina euro 3.1L with 80k miles my problem is I cant get the heater to work when I am driving. I live in upstate NY where we get sub zero temperatures. When I let the car idle for a long time it will warm up perfect but as soon as it after 20 minutes it will blow freezing cold air again. If I pull over and let it idle it will start to warm up again. I checked the antifreeze level and loosened the bleed screw and no air came out of the system, I also checked my heater core and that seemed to be ok. Any help would be great.

It's sounds like your thermostat may be stuck open.

I was thinking that but never i herd of them sticking open just sticking shut i guess i will pull ot and see.

Most of the time they do stick closed but occasionally you get lucky. Some thermostats are designed to stick open. They're called Fail-Safe in case you'd like to use one.