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.