when overheating occurs at highway speeds the cause is usually. a radiator or coolant circulation problem, Check for a restricted or clogged radiator.
The DeVille could have low transmission. Or the internal transmission bands or valves could have failed. The transmission will most likely need a rebuild.
uotside temp sensor in front of the radiator has gone bad
There are several things that can cause your Cadillac Deville to have transmission problems. The most common transmission problems are worn transmission gears.
O2 sensor, and catalytic converter can be ruined. You can also cause internal damage to the engine, depending on what the problem is. Have it repaired.
check radiator for small leaks or cooling lines which would most definitely cause it 2 steam when running after awhile
1 of a 100 but the 1st to check . BAD THERMOSTAT
I daily drove mine in Iowa in the summer and winter from 2005 to 2011 and got a fairly consistent 9 mpg.
Heater core has failed.
dirt in the speed snsor bad speedo head in the dash