I've got a 1998 Deville and it has overheated on me twice. The first time was a long while ago when it was going up a long and steep hill; I let it cool down and just kept an eye on it since, but then winter came and the outside temperature became more cooperative to keeping the car cool.

Now that it is getting hot out, the car has overheated on me again. Here is the situation:
Since the first time, I have been watching the coolant temp and turning the hot air on to make the fans kick on to cool the engine. It seems that this is the only way to get the one fan to turn on. With the heater blasting, the fan, I believe on the passenger side by the radiator, will kick on and drop the coolant temp to about 210. If I don't turn on the heater, it seems that the fan will not turn on and the temperature will continue to rise.
This most recent time that it overheated, I could not get the fans to turn on. I believe that the reason was that the car was reading the outside temperature as 87 degrees, and the heater only goes up to 90 degrees. I simply had to abandon the car until the sun went down.

Has anyone experienced this? Does anyone know what might be causing this problem or how to fix it?

Need more accurate info. The radiator fans are not controlled by anything except the coolant temp. Blasting the heat in the cabin has no effect on them. What's the coolant level like? Constant or are you loosing it. What do you mean by your engine over heating? Did it tell you or are you thinking it just got too hot. Do you have any codes? Search the forum to understand, come back with more info to get help. K_C

It seems like I'm losing coolant, but it hasn't been terribly drastic. I've been refilling it every couple of weeks.

When the engine overheated it told me that the coolant was low, the temperature light came on, it told me to idle the engine, and the coolant temperature hit almost 260. It has only happened the two times when I was going up steep inclines and had to put the peddle down.

I tested the fan theory a while back. Everyone always tells me that, when an engine is getting hot, you can turn on the heater and it cools the engine down. I left the car sit on a hot day with the engine running and watched the coolant temperature rise to almost 230. Every time I turned the heater on, the fan on the radiator would kick on and the coolant temperature would drop to 210. The fan stayed on whether I left the heater on or turned it off, but the fan only seemed to turn on when I turned the heater on.

I don't have any codes. The service engine light came on when it overheated, just like the last time. Previously, I had to reset the oil life and disconnect the battery in order to get the service engine light to turn off.

I searched the forums before, but didn't find anything that sounded like it fit my problem. Of course, I'm not sure exactly what the problem is.

Sounds ALOT like a headgasket that is slowly getting worse. I would make sure it isn't before you put any money into it. You can go buy a kit (very cheap) that will test your coolant for exhaust gases, or its very very cheap to have any repair shop do it. Another low-tech way you could check is to rub the inside of your tailpipe with your finger (when its cold) and see if there is any coolant in there, it will be slimey. If it passes the exhaust gas test, have it pressure checked for leaks, since you are losing coolant.

:yeah:

If the DIC lit up you more than likely have codes. Have you followed the instructions in the "How to pull codes" sticky at the top of this page? The dash will then display "current" and "history" codes, write them down and come back. You've got something going on there. Sounds like you're underpowered on that up hill climb. K_C

I checked for codes and this is what I came up with:
P0300 - Engine Misfire Detected
P0463 - Fuel Level Sensor Circuit High Voltage
P0603 - Control Module Long Term Memory Reset
P1258 - Engine Coolant Over Temperature - Above 268 Degrees F - Protection Mode Active
U1255 - Class 2 Communication Malfunction (Serial Data Line Malfunction)
U1064 - Loss of Communication with DIM

I haven't tested for coolant in the tailpipe or exhaust gases in coolant yet. I should be able to do that in the next day or two.