My 97 Cad. Deville overheats when I exceed 50 miles per hour. When idolling it is ok. First diag. was the exhaust covertor and I had it replaced. Didn't work. The second was the radiator, although it is two years old, so I had it removed and flushed and a new thermostate installed. The car continues to overheat.

 

I had a 1994 eldo with same promblem. 99% of the time its the head gaskets have gone bad. Take it to caddy the can run test. Warning very expensive job.

 

check your heater core hoses when your car is at operating temp. if one is much hotter than the other one your heater core is plugged. I had 1998 deville 4.6 that no one could solve the overheating problem in after changing water pump, radiator, thermostat, had it flushed it still over heated. everyone said it was more than likely the head gasket, but it wasn't using fluid. as my last resort I unhooked the lower heater core hose and bypassed the heater core to see if it would solve my problem, IT DID ! For some reason if the heater core is plugged it doesn't allow the coolant to circulate properly in the engine, causing it to overheat. You can bypass the heater core by removing both hoses off of the heater core and connecting them together with copper pipe or plastic coupler, it's the easiest way to see if it solves your problem, if it does then replace your heater core.

 

While it can be the head gasket, I do not think the probability of that is anywhere near 99%.

There are many factors here we do not know. Is the system losing coolant? Has a pressure/leak test been done? I'd check the lower radiator hose; it might have gotten soft and collapses when coolant flows through it rapidly at higher speeds. The water pump itself can be bad, or its belt is slipping...

I'd start with the simple things, diagnose it accurately and go from there.