Cadillac 99 deville

Overheating can be caused by several issues. Low on coolant, leaks (internal or external), clogged radiator, bad thermostat, cooling fans not running, etc.

I'm not sure if you ever found your problem. However if you did not, what else you can do is to check the water pump belt and tensioner. I had to replace the tensioner a few times causing an overheat condition. Also check the purge line and valve. A clogged purge line will cause and overheat condition causing no escape for the air build up inside the cooling system. And lastly with the thermostat, with any new thermostat you buy, you must compare it to the original that came out of your car. I came across a very common mistake of new thermostats being smaller around not giving enough flow for coolant. 180 degree is the best to have especially if you live in a hotter climate. Hope this helps.

Best answer is to get a head bolt kit. It comes with longer head bolts. This particular year 1999 deville is what I have. The factory dis not put long enough head bolts on these cars and they wear out. The kit comes with bigger head bolts and adapters to put them in. Ask google for 1999 cadillac deville head bolts kit. They were supposed to recall them but did not. 100% fix head bolt kit for 1999 cadillac deville overheating problem.