Engine Cooling problem
2002 Cadillac Deville V8 Front Wheel Drive Automatic 83000 miles
I recently bought a 02 deville and right off the lot b4 I could get it home, a message appeared saying that the coolant was hot. When I finally got it home and let it cool down I discovered there was no coolant in the reserve tank along with some black sludge. I refilled the tank but it still overheated. I took it to the used car lot I got it from and they said it was the thermostat. They replaced it but it was still overheating. They then told me that it was the thermostat seal and that the guy would not be there until monday(this was a friday). I only live a few miles from work so I had no problem driving it over the weekend. These past 2 days i've been driving it, it did not overheat but the coolant is burning off after only 10 or 15 miles, and the black sludge is a concern for me. There are no leaks so I really dont know whats going on. If anyone could offer any advice on what the problem could be or give ur opinion on whether I should keep the car or give it back and get another. It does have a 6month warranty. Please Help!