What will be causing white smoke from tail pipe,and no coolant in radiator. on 1997 GMC Yukon

Last couple of days now, gage light will come on,water pressure running hot, but then will go back to normal,heater takes long to work.twice I had to add coolant.I checked for leaks,radiator,hoses, around engine,no leaks that i can see,no coolant on ground either.

Asked by for the 1997 GMC Yukon
2 answers
You need to stop driving this truck or you will be replacing the engine. You have a leak in the cooling system. You need to find a repair shop to check for a leak. By now you need to have it checked for a blown headgasket.
Agree with Parts Guy! Stop driving the truck, having the cooling system pressure tested for leaks, it may be allowing coolant into a cylinder due to a failed head gasket. There is a simple chemical test to look for the presence of carbon monoxide in the cooling system (which would only be present if the h/gasket was blown), the test is called a Block Test.