It is, and has been seriously plugged or broken. 6-months ago, on the way to work, I smelled antifreeze badly and when I turned on the heat it was even worse. Then I noticed that my temp gauge was up 1/2 a tick. Concerned? Yeah, but it wasn't overheating. Then I also noticed that the passenger side floor board was soaked with anti-freeze. When I followed up and read through web explanations and this particular site, I saw that it sounded pretty much like what had happened. Now move up in time to last month. I got clipped by a hit-n-run driver that just grazed my front bumper from the side, but enough to take my bumper off and crack my radiator. Put the new Radiator in (with some help) and within 5-minutes of running dark sludge and antifreeze started spilling out on the passenger side. Car overheated quickly and I shut it down. Yes, I know that there are many working parts, but with a new radiator? That's probably not it. And I knew that the heater core was already causing me grief. Should I try to flush it somehow first before I go to the extreme? I've got the new heater core unit in the garage. Any thoughts? Thanks for your interest. I'm trying believe me. thx, B