So I have an 01deville and it has almost 100k on it. I bought it 2 months ago used and it started overheating after a week. Had the system purged manually and it worked fine.... for about three weeks. Then overheated again. Looked at coolant, it was about half a gallon low so I added some. Ran fine. Then I replaced the surge tank and it was good. Then today I parked it on a hill for not even 5 min. When I came out the DIC was going crazy. "Engine coolant hot- idle engine" "engine hot - ac off" "change engine oil" (which is weird considering I just changed the oil 3 weeks ago). There was No ac Running and I let the engine idle after backing the car down off the hill. "Engine overheated - stop engine" so I shut it off, let it cool down turned it back on. Drove a mile and a half and it overheated AGAIN. Had it towed home and here I am. Someone, anyone, ideas?
The correct thermostat - AC Delco, Gates, Stant, etc., etc. - is set to begin opening at 188 and fully open at 206. Most Northstars run at 190 to 200 or so, so the thermostat is normally only open 10% to 75% even in August hauling down I-95 at 85.
The radiator end tanks - plastic - tend to crack at this time/age. Replace the whole thing if you have tank leaks. There is a 99.9999% chance that your base Deville does not have V03 HD cooling, so you want a radiator without the engine oil cooler in the passenger side end tank. (IF the car was built with the towing package and V03 that would show on the RPO sticker under the spare tire cover.)
Worried about actual coolant temperatures in these models with no real temperature "gauge" and OBD-II ??? Look at a Scangauge-II. www.scangauge.com - Fun tool for maintenance, engine operation and diagnostics.