1998 Deville Overheating problem
 

My 1998 Deville keeps overheating. I have tried everything to diagnose the problem. Checked purge line. Pressure tested for leaks. Inspected hoses. Water pump drive belt and pulley. It's going through coolant like crazy (1-2 gallons a day). Car is in great shape in all other aspects. Only has 73k on it.

The car starts normally. Drives fine for around 30 miles in town. Temps start out at around 196 and stay there for about 20 miles. Then the temp starts creeping up. First 201, then 208, 212, 217, 222, 228, then drops down when the fans kick on to 206. Then starts rising again. does this a few times and then just keeps rising until 235, 240, 244, 248, etc. By the time I get that high I stop and let it cool. Check coolant level warning comes on. I check the level and add sometimes 2-3 gallons. The whole thing starts all over again. I just tried to block test the engine and it pushed coolant into the tool but didn't turn yellow. Started leaking out the top of the block test tube like a fountain. I noticed a leak on the pump housing that mates to the block right under the egr valve. White residue all around the area, doesn't seem like that would account for that much coolant loss. I plan to go ahead and replace the water pump and the tstat. While I'm doing that I'll replace the lower hose (already did the upper). I'm at a loss. I've driven the car from Colorado to Oklahoma several times. overheated in Kansas once since then not much then all this started. Wondering if I need to replace the radiator or am I just throwing parts at this? I'd like to diagnose prior to spending a ton of cash on stuff that won't fix the problem. I am refilling with plain water every time since Dexcool or anti freeze would be killing me with the loss of so much coolant.

I've searched the posts on this site and others. Haven't found a solution and need some help from you guys......Please, pretty please help me!!!!!!!!!!!!!!!!!