What are the major signs of a bad heater core? I've suspected for quite some time that mine is going south because it started to smell like exhaust whenever I turned on the heat. Lately it's been exceptionally cold in Columbus, OH and my car has been running very hot almost to the point of overheating. The only way I can get the temperature to go back down is to turn the heat on and off until warm air comes out and takes heat out of the engine bay. Has anybody else experienced anything similar to what I've said? It's important to mention that I installed a Mishimoto radiator and a new OEM fan about three months ago. I can't imagine that the radiator or the fan has gone bad already. I have a small coolant leak on the driver's side in the front as well. I suspect that it's just a cracked hose though. I haven't been able to investigate it due to weather. If anyone has experienced this or knows a solution, please let me know.