I have a 2000 blazer with the 4.3. The heater has never been what I would call hot. But the last few days with the freaken cold weather it's been cool in the truck. If I let it run in the driveway it gets warm, then if I drive on side street it stays warm ish. When I go on the highway it goes cold. The temp gauge stays about mid way (normal).I've never worked on the cooling system on this truck,I got it in the spring. Any ideas?

 

same here. I'd bet your heater core is plugged. That was my problem. Even flushed, it's still not 'hot' though.

 

I'll have to take a look tomorrow,this dam cold weather,I hate winter. Did to make any differance? I had a 2000 dakota and man the heat was so good I could cook an egg on the dash 10 mins after it started.

 

im having the same issue with my 2000 gmc jimmy. ive tried to replace the vacum hoses in the engine bay,added a vacum canister to mine,changed the vacom control silanoide behind the dash,replaced both actuators beside the drivers foot and in glove box. replaced heater cotrol panel and no luck. i also run green coolant,changed to prestone thermostat and it still has same issue. heater core isnt plugged i ran a hose through it and its still the same issue. i only get dash vents or window vent working at the same time no matter what selection i change it to

 

I can change from floor to dash to defrost. the temp is still the same,I plan to look inti it some more this week.