There are a few things that can cause this problem.
1. Check your antifreeze level and make sure there are no leaks in the cooling system.
2. Check the cable that connects the temp lever to the heater control valve. Make sure it's connected on both ends.
3. Check the heater control valve to make sure it works properly.
4. Check the thermostat and make sure it's not stuck open. Do you get any heat at all? If you get heat only when the car is fully warmed up and at a standstill, but not when you are moving, then the chances are you have a bad thermostat.
5. One last thing. Make sure no one bypassed your heater core. If the heater core was leaking at some point, and someone wanted a cheap fix, bypassing the heater core is the cheapest fix and the car will still run normal.
Just a bit of advice, don't forget to burp the system once you get it back up and running! This was covered somewhere in the forum but I haven't found it. You'll have the same problem, or worse, if you don't!
I think he meant bleed the system after you are done flushing it. Fill the cooling system with 50/50 antifreeze and run the car with the radiator cap open until the car reaches normal operating temperature. Make sure no there are no more air bubbles, then cap it. Make sure the overflow bottle is filled to the max line. Use water to rinse off any antifreeze you might have spilled and recheck the overflow bottle in the morning when the car is cold. If the overflow bottle is low in the morning, fill it to the max line and you're all set.
Swapped the thermostat today, filled it up with water and warmed it up. Still blows cold . I'm gonna flush it saturday and keep my toes crossed. I just have this sinking feeling that someone bypassed the heater core :bad-word: but we shall see.
If someone bypassed your heater core, then you should be able to look under the hood and determine it pretty quickly. If it is bypassed, fixing it might be easy, and it might be more involved. It depends on what the previous "muh-kan-ik" has done to the car.