citreon zx air in water?

right then,i got this shed of a citreon zx desiel.from cold with the rad cap off its bubbling away.loads of air in it.if i undo the bleeder screw,it air comes out there like funk.it looks like it washing up liquid.super throffy.i tryed to bleed it up,was left this ratter running for an hour but it didnt get any better at all,im begining to thing the head gaskets blown & its just pumping air straight in through the cylinders.anyone know if it can be caued by anything else that a blown haed gasket?