2000 Jimmy Blowing warm to cold

I have a 2000 Jimmy and the heater suddenly will blow cold air, sometimes it blows hot, somtimes it blows luke warm. It blows, but the air is never always hot. Any ideas... Need more info?

Mine's Doing The Same Thing--i Noticed It Would Get Warm At Sustained Speeds Of 70-80 Check Out The Post Re: Do I Need A Reverse Flush On Heat System? I Haven't Tried It, But I Am Going To.

sounds like a problem in the heater control valve...you may have crud built up like the lat post ...or may have a vacuum supply issue to the valve...

Mine was doing something similar to that. I recently flushed out the Dexcool really well and replaced it with 50/50 green Prestone and the heater has been working much better so far.

My 98 Jimmy was doing similar. I had changed the antifreeze last fall the radiator was very clean, so I didn't flush it. Recently the heater would not blow hot air. I noticed the thermostat was running about 160 instead of normal 195 so I replaced it, but that didn't fix the heater problem. After reading through the post on Automotive Forums, I decided to flush the heater core with a water hose. That fixed the problem. The heater will get as hot as you want now. Thanks to all who post problems and successes here.