White smoke coming from tail pipe

White smoke is a sign of burning antifreeze, typically due to a leaking cylinder head gasket. There is a chemical test that can be performed to verify that.

Also, GM has had plenty of trouble with leaking intake manifold gaskets on some of their engines. If coolant gets sucked into the intake, it will be burned in the engine and go out the tail pipe. That is a less-expensive repair than a head gasket.