92 Previa with air bubbles in coolant tank

More likely is a leaking head gasket. Lets not hope for the worst yet. Your mechanic can do a "sniffer" test at the radiator to verify you're seeing combustion gases. He will draw air through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are there, the liquid will turn bright yellow. He can also perform a cylinder leakage test. That will verify the leak and identify which cylinder is leaking.