My 2000 Cherokee has developed a strange issue. My heat varies by speed. When in stop and go traffic, it's blowing cold air. At moderate speeds, it's barely warm, and at steady highway speeds it will still roast me out of my seat. Even if I slow to a crawl to take a turn and speed up again, it will get cold and then hot again almost immediately. The fan still works fine (well, on speed 4 only- probably the stupid resistor again). I've been taking highway route whenever possible to stay warm! Engine temps are normal, coolant seems fine. I did have one issue where it overheated a few months back, due to a friend who borrowed it and left it idling for at least 20 minutes. Last time I ever lend a car out to someone in need. I doubt they are related, but who knows. I searched, but couldn't find anyone with the exact same heat issue, and there seem to be no shortage of them with these Jeeps. Any suggestions?

Idk i'm guessing but maybe your water pump isn't feeling too well.

sounds like you've got some clogging going on... Back flush that heater core, and then flush the whole system.

there is air in your cooling system. fill it up with the cap off the radiator, turn it on, let it bleed out, with the nose pointing uphill.

Thanks for the advice. I just drove an hour in 20 degree temps with NO heat- it decided to stop working altogether after a month of working on and off. 10 minutes from home, my temp gauge shot up, and then back down again as I was pulling over, then repeated that. I will check and see if I have an air bubble in there, and failing that, try to flush the whole thing. Hopefully it's something minor- if not, bye bye Jeep- this is just the tip of the iceberg in terms of recent issues I've had.

your friend that overheated it had coolant boil over, and it never got topped off afterwards. not enough coolant loss to cause major overheating, but enough to cause heater issues. it's air in the s ystem, trust me.