i think i put a video up of when this happened to my chevette. you get what looks like smoke out the exhaust but is fairly white. It is water vapour basically.
The most common failure on the 1256 is the head gasket , especially between cylinder 4 and the water jacket. As a high mileage driver of these beasts i have had that happen three times now .
They also fail from age as the gasket thins, the head tension reduces and coolant seeps into the gasket, rotting it. the combustion chamber gasket rings also rot away, exposing the gasket to the full force of compression.
The last HG change i did i took heed of some advice i found on google and before fitting the new gasket, i brush painted it with wellseal, a liquid gasket compound. far better and easier to use than hylomar.
you brush it on both sides like paint (use a good paint brush or non fluffy sponge etc..) , let it evaporate its solvent a while then fit it to the car. It is sticky stuff so avoid getting dirt and crap on it before fitting too.
I used one of those household paint / paper removal scrapers that you can drag backwards (has a rigid handle/blade) to clean up the head and block surfaces too prior to fitting.
so far so good some 10k miles+ later
Interestingly the original replacement GM gaskets for the chevette were also coated in a sticky stuff so it seems this was the done thing .
the wellseal should keep the corrosive/damaging coolant and oil away from the gasket and as it is non hardening should keep the head sealed nicely .
I also initially torqued the head evenly in stages from 60nm to the recommended 76nm (i think) and went past that to 80 as a final.
Make sure the head bolt holes are clear and the head bolts threads are clean too. brake / clutch cleaner aerosol helps here as does pressurised air
Also make sure no bits of old gasket fall into the oil way that goes from block to head . stuff some rag or kitchen roll in that hole while working on it.
if crap in there gets pushed into the head it will block the small oil ways that lube the rockers.