When It's cold out I'll Start the car and let it run for about 5-10 Minutes with the Heat on full blast. I'll get in the car and it'll be up to or almost at running temperature, but it'll still be blowing cool air. It seems like it's a waste to run the heat cuz it'll just be colder or I'll already be where I was headed by the time it warmed up. What could be causing this? It seems like last winter if I remember correctly the heater worked great or I just thought it did.. (upgraded from a 95 v6 with no heat lol) This past summer I got a Low coolant light and it was full of coolant.. so I got on here and some ppl just disconnected the sensor to make the light go away, which I did thinking it was just faulty. For some reason I'm thinking it could be the thermostat? Any help would be appreciated.

 

The light comes on because the sensor at the bottom of the overflow tank sticks. It gets gummed up. Take the whole tank out and flush it with some Dawn/Hot water.

Check the Blend-door connections on the back of the HVAC panel. Should be rod operated. If you move it by hand, you should hear the door open and slam shut. -Varies how much air is being taken from the outside or from the heater core.

Either that or you have a stuck thermostat. The 98 was the only LS 4th gen with a REAL temp gauge. The others just sit right at 200 when the car is "warmed up" regardless of actual ECT. So the car may not be fully up to temp, but the gauge will show it at operating temp.

 

Clogged heater core