1998 Cadillac Deville overheating . Engine overheats to 245 + degrees when running 50 mph or more with some torque load.( ie.hills and 4 passengers) New thermostat and coolant reserve pressure valve.were installed. Engine runs fine for 1st 20 minutes or so, then temp rises to 220 + degrees at speed up to 50pmh or so. Temp goes back down to 199 or so when in regular , slower traffic.

The most common overheating problem with the Northstar engine is a blocked purge line. It's job is to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.The hose goes in to the engine to a crossover and comes out the other side to near the thermostat housing,the blockage may be there.

I have the same year and model. I was having the same problem, I replaced the water pump, some leaking gaskets, and also the thermostat. 2 Days later I had to drive about 75 miles, then the damn thing over heated again. I checked everything to find out that one of the 2 cooling fans is only funtioning on low while the other is working properly on both low and high. hope this helps

What size themostat was installed temp rating?

Dirty Radiator,worn water pump (they get eaten chemically over time) Ignition not working correctly. Cooling Fan,not comming on. Pretty much a list of things. One at least is not working right