1994 Chevy Camaro

If you have the LT-1 engine, then you have a reverse-flow cooling system.

Whenever air is introduced into those systems, it will collect in the heater core and stop the coolant from circulating correctly.

If you rev your motor past 1500 rpms with the heater controls set on MAX, then you should feel heat if this is the problem.

If not, or you have the v-6, then it's your temp control valve that will be located somewhere along the hoses that run from your radiator and heater core. It can also be a blend door within the heater core containment unit. This will pull in cold outside air instead of blocking it off.