1991 Cadillac El dorado, Overheating problem

[quote:59ff25ea90="draftsman0"]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.[/quote:59ff25ea90]

probally head-gaskets or a leak else were, is there coolant in the oil? also; does the fan up front run, has 2 speeds