Blue Devil is good for a blown head gasket or even a
cracked head as a band aid. But you have to follow the
instructions exactly to the tee! The fact that your heater
is not working is a classic sign of the cooling system
passages in the engine being clogged and the heater
core is probably clogged now too. You need to flush the
cooling system with a chemical cleaner to get all the
rust and junk out of the system. Be sure to flush the
whole system, not just the radiator. Also, after the flush,
when you refill the cooling system be sure there's no air
trapped in the cooling system. That will also cause the
heater not to be hot and the engine to overheat. To
check for a blown head gasket use a cooling system
pressure tester with the engine cold. Pressurize the
system and let it sit for 15 minutes. If the gauge doesn't
drop within that time you have no leaks. Now, with the
system still pressurized, start the engine. If the needle
on your pressure tester gauge bounces with the engine
running, the head gasket is blown. No bouncing, no
blown head gasket. Hope that helps! Jim