^ +1.
compression leaking into water jackets.
do a compression test to identify which cylinder(s).

 

Had the same thing happen to my 97 Lumina. Exhaust started getting pumped into the cooling system. popped a few holes in the hoses and filled a few gallons of tap water up so i could take it to school each day. Worked for a good month before it got too cold and the engine seized up. Get those heads machined or check for cracks. otherwise you're SOL