'83 zx heater

Auto air or manual? Chances are the vacuum lines running from the vacuum canister by the inspection lamp has a crack in it. The crack is usually right at the canister. Sometimes you can snip the end and it my last a little while. If you see the crack and snip the end you can test it by sucking on it with the heater on.

If it is stuck all the way hot, it depends on if it is auto air or manual. Auto air has a vacuum hose that controls that function and defaults hot. Manual is controlled by a cable.

its a manual heater.

Then either the cable or the valve itself came open. You can bypass it outside at the block temporarily. At least that will stop the heat.

Ok, Ill check it out. Thanks dude.