My 1991 Cad. El do, 4.9L 174,000 mils overheats, it starts fine and can idle for over 40mins. Without overheating. Once on the road it climbs to 260 degrees and I have to pull over. I had the engine flushed, coolant pressure checked and flushed, changed the water pump, thermostat, and heat sensor. Still it overheats. So I tried the blue devil with water and it seemed to work until I replaced the water with coolant. It still overheats while on the road.