1994 pontiac firebird HAS NO HEAT

This sounds like a thermostat that is fully open all the time. It allows the engine coolant to warm when sitting still (no air being forced thru the radiator), but even that goes away when moving, since the air through the radiator will remove that heat from the coolant. Does you van have a temp gauge? Is so, at full operating temp. most vehicles have coolant temps of 180-195 degrees F, depending on the thermostat installed. Anything less indicates a thermostat that is starting to malfunction.

Could be air in the cooling system , try an bleed the system . On top of the thermostat housing is a bleeder valve , with the engine up to temp. open valve till coolant comes out . Is the coolant full ? What color is the coolant ? Have see where coolant turns like a muddy color an clogges the heater core an engine coolant passages . Check the heater hoses going into the fire wall ,are they hot ? Does your grand prix have manual or automatic climate control ? Duel zone ! Do you hear a clicking sound behind the instrument panel ? Bad temp blend auctuator .

I've had this problem. 2000 GMC uses a closed coolant system. Meaning takes and returns coolant from and to the Reserve Bottle and has little air in the engine and heater systems.
If everything appears ok but no heat. "Air Lock" is the most likely cause. (With interior heater set to full heat) Solution; 1. Make sure water pump works. 2. Drain about 1/2 engine coolant into container. 3 Remove both heater hoses from engine side and flush heater core in both directs through heater hoses with garden hose. 4. Reattach heater hoses in correct locations. 4. Refill engine with coolant. 5 Start engine and run for 2 min., use caution when topping up coolant.
6. Most 2000 GMC have twist off caps with plugs located mid way on heater hoses. with engine cold, start engine these caps may be loosened to allow trapped air to escape. Do not attempt with engine hot due to hot coolant injuries. This may take more than one attempt to remove all air from heat system. 7. Lastly most 2000 GMC water pumps have a bleeder plug,( 3/8" square driver plug) located on the water pump. With egine cold this plug should be removed to remove any air. Reinstall plug with thread sealant.
Hope this helps cheers

If no coolant flowing through the heater core then ther is no heat. There are 2 possible root causes needed to check out: 1- THE VALVE TO CONTROL THE COOLANT FLOWING TO THE HEATER CORE IS NOT OPENING AS DESIRED. 2- THE HEATER CORE GOT CLOGGED UP AND NEED TO BE FLUSHED.

my 2004 pontaic bonneville the heater hoses going into and out from the heater core are hot , but the heater blow cold air. ,

Are U sure the coolant is full and all the air is out of the cooling system? if the engine was badly overheated then u may have damaged the cylinder head gaskets, this can cause the cold air as well as low coolant..

It sounds to me like you're low on coolant. When you checked the coolant level, did you actually open the radiator cap and make sure the radiator is full? The plastic tank you generally add coolant too is actually an 'overflow' or 'reserve' tank. There is a hose that runs from that tank to the top of the radiator so if the radiator gets low on coolant, more coolant -should- run in from the overflow tank. However in practice this does not always work. I have a Jeep that leaks coolant and experiences this exact same problem. Coolant will not run from the reserve tank into the radiator because the hose is plugged up, and whenever the radiator gets low on coolant the temp gauge will go real high and then drop, and the heater doesn't work well. This is because the cooling system of your car is supposed to be a closed system, full of coolant and no air. When coolant leaks out, the space it used to occupy is now occupied by air, which does not transfer heat well. When 'air' is passing through your cooling system, no heat can be transferred from your engine to the heater and radiator, resulting in a hot engine and no heat at the heater. Then when a pocket of water passes through the system, the temperature gauge quickly falls as the water absorbs the heat from the engine. The hot water that cools the engine is where the heater gets it's heat from as well, so when water passes through the heater core, the heater works, but when it's filled with air, it doesn't.

Help is HERE!
It looks like an Air lock scenario!
Park vehicle on level ground, when cold remove coolant filler cap, start engine and leave to idle, turn heater to hot blower on full. When at operating temp keep clear of coolant filler area and listen, hopefully gurgling then a boil over, wait and then top up with very warm water/coolant let settle it may boil over again whilst air is still in system.
Check for heat inside car, if hot and coolant settled replace cap but keep an eye on temp gauge for a while as air lock may still be further along cooling system.
Please press the Blue button if sorted!
Paul 'W' U.K.

check the coolant level and make sure you have enough antifreeze and also the thermastat may need to be changed but check the coolant level first and also let the car run to temp while adding coolant to make sure you get all air bubbles out because the thermastat will not open if there is bubbles in the system

first of all I would make sure the car has proper amount of coolant and you are right it is very possible you have an air pocket in the system also ask your self did you change the thermastat after you did the heater core when the car is cold make sure there is coolant at the cold mark on the reservior bottle because you might not have enough coolant then start the car and get it up to temp and if the coolant goes not go up to the hot mark when the car gets to temp you do not have enought coolant just keep adding coolant and you will see the bubble after the thermastat opens