1999 cadillac seville sls

overheat when driven not when in park it idle normal tempature and cooling fan turn on at 228 degrees and the surge tank overflow

maybe thermostat, is there any more info you can give us?

replace the t stat

your tank fill tank may be too full

228F is the normal temp for the fan to turn on. These cars operate normally 205 to 215F which is about 12 o'clock on the gauge or just a tad past the needle being straight up. Its unfortunatly common for these cars to experience head gasket failure and to have the coolant tank tested for ex gas would be in order. this sounds too familiar but you need to check things out further. This repair to correct the head gasket failure may determine the fate of your car due to the expense vs value of the car. Its typical for the temp to climb well past the 12 o'clock under power with a failed head gasket and have normal temps at idle.

I to have had the same overheating problem. It turned out to be the head gasket. The bolts kept comming loose. The dealer had to replace all of the head bolts. Just replacing one won't fix the problem. Luckily I had a warranty as it was $5,000.00 to pull the heads. After 4 times in the shop for the same problem the insurance co. changed all of the bolts. steel to aluminum does not mix. Hope this helps. 99catty