2004 Chrysler Sebring Sebring blows hot at high RPMs, cold

I have the same car and have been having the same problem on and off for the last year. Check your antifreeze level.

I added some and the problem completely went away. I didn't notice it all summer until now. Only this year it was worse.

The problem is that I kept loosing coolant. I wrote it off a small leak (no big deal) until it took my thermostat. After having it checked I found out that the coolant resovoir had a crack in it. That's why I was loosing coolant. I recommend if you have any problem like that, fix it soon. The thermostat is cheap to buy to actually change it will cost you a lot of money or a lot of frustration. It isn't easy to get to.

You're right, this seems to have been the cause of the problem. I did check the coolant level when the problem cropped up, and it was within spec - but very close to the "low" line in the reservoir. I didn't have any of the proper coolant type on-hand, so I left it as it was. When I read your response, I picked up a bottle of coolant, and filled the reservoir to the "MAX" line (about 1/2 quart was added). The problem with the heat cleared up immediately. Interestingly, when I checked the coolant level again the next day, it had dropped back well below the max line. I jacked up the car and checked for leaks and found nothing. Right now, it seems like when the coolant level gets low in the reservoir, the actual level is lower than it indicates. I'm wondering now if the coolant level in the reservoir needs to be much higher than the "low" level in order for the cooling system to be able to pull fluid from it. Nevertheless, I'm happy that the issue seems to be resolved for now, unless there's a leak that I haven't been able to detect. I'll definitely be keeping an eye on it. Thanks again for sharing your experience with this problem!