This began with a low coolant light on my 96 850 GLT. When I checked the overflow tank, I noticed a ring of crud around the opening after removing the cap. The level was just barely triggering the warning lamp on turns. I topped the fluid, but am now experiencing low heat from the climate control system. The engine temp pretty quickly gets to the normal position and stays there. So it doesn't sound like a thermostat problem. I have never had an indication of overheating. But when I topped off, I screwed up and used undiluted antifreeze. Could it be something as simple as too little water in the cooling system? Or is something else going on?