2002 Deville base. I was driving up mammoth mt. And temp which was normally always at the middle. Then it overheats. And I pull over to let it cool. Temp got all the way up to max hot. I let it cool and add about 1.5 gallon of water. It overheats in another 10 miles at 50 mph. I pull over to let cool again. I drove really slow at 35 mph And this time it did not overheat. I'm really scared to drive home. It's about 500 miles home. I open the coolant cap this morning and I smell exhaust gas. Any suggestions?