I have a 93 Z28 with no heat I replaced the thermostat but there is still no heat and some times the temp gauge gos all the way up and then drops and some times smoke comes out of the defroster vent

 

Your heater core is shot and your fluid is low that y the temp gauge bounces around, or maybe not not that low but you need to bleed your system. but it your heater core is leaking and steam is coming threw the defroster bleeding is kinda pointless till you fix the heater core.

 

Thats what I was thinking but there is nothing leaking in the passengers floor board and the low coolent light doesnt come on butabout once a month and it takes about a 1/8th of a gallon of coolant

 

your heater core might be all clogged up where the leak is and only steam can escape from it but im sure the heater core is a goner

 

Well a new one is $60 I just hope its not a pain in the a$$ to change

 

odds are it will be :-(

 

People have posted that changing out the core is not real hard. There are very few steps involved.

 

Good Luck :-)