Monte Carlo overheating

Of course this could be 100 different things.
Any way, if the heater is not blowing heat it usually is because the fluid level is low or the heater core is clogged. If I had to guess from every thing you have stated I would guess your cooling system is clogged or restricted both in the radiator and the heater core.
A quick way to tell if the heater core id clogged is to fully warm the car up and turn on the heat. Touch the two heater hoses that go into the firewall. If one is very hot and the other one is not hot the heater core is clogged. I would also assume that the radiator is clogged.
Hope this helps

Could be a thermosstate

Hello: Hmmm no heater means no hot water circulating. If the engine runs the water will get hot within a few moments depending on temp of the engine and the size of the engine. The thing that moves the water is the water pump. If there is an air pocket in the vicinity of the water pump impeller the water will not be circulated through the engine. If the water pump is not turning no water will be pumped through the engine. If the thermostat is not opening at its temp (probably 190 deg or so) water will not circulate. If the water pump impeller has broken loose from its pump shaft water will not circulate. If the radiator is plugged, water will not circulate. So as you can see there could be several things causing your problem. Air cavitation could even be caused by a air bubble being pumped into the cooling system through a defective head gasket even?
Good luck
Darryl.

How do you test for a leaking head gasket. It's like it keeps getting air in the cooling system bleed the screws off and its fine for a while.

HELLO: Well a blown head gasket will cause blowby into the water jacket at time of combustion, this will cause air to be pumped into the cooling system. A slight leak it will simply take longer, a big leak and engine will overheat quiet rapidly, Plus probably blow most of the coolant out of the radiator. The cooling system should be a air tight sealed system using an overflow recovery bottle. If the radiiator cap is in good condition as the coolant expands from heat it will go into the recovery bottle. As the engine cools the coolant will be sucked back into the radiator. If the system is not air tight this will not happen. You could pressurize the radiator with a pressure tester and see if it bleads down by pushing water into the affending cylinder. However if it is purely a high pressure leak this may not detect it. You could pull all the plugs, pressurize the system and see if it bleeds down (hopefully pushing coolant into the affending cylinder) Crank the engine and see which plug hole the coolant flies out of
good luck
Darryl.