I started my car this morning to let it heat up and start blowing hot air. Well 15 min later I got in and it was blowing cold air, I checked to make sure the heat was turned all the way to hot and made sure it was on outside air, even tried the inside air. I then shut off the car and unscrewed the radiator cap and it was blowing out hot green water so that was ok, my oil was full, the only thing was my little resivoir to the left of the radiator was slushy so I poured a little hot water in and then around the outside to make sure it was thawed. But since I had to go, I had to **** it up and drive, I drove about 15 miles and not once did any hot air come out. Any suggestions? This is the first time this has happened. Also there is no moist water or anything on the floors so the heater core looks good.

Interesting symptoms.If the expansion reservoir was filled with icy slush, then your coolant/water ratio is way off. My first theory is that your cooling system needs a thorough flushing.

Heater cores can get blocked with rust (from the engine, when there's too little green coolant in the mix), as can the radiator.

The solution requires the cooling system to be drained. The heater hoses should be disconnected and back-flushed with a hose. Replace the radiator and heater hoses if they are soft and squishy or very rigid. Replace any hose clamps that need it.

Remove the thermostat and check it's not stuck open as this will cause the no-heat problem. Replace it if there's any doubt.

Put everything back together, fill with soft or distilled water, bring to operating temperature. Let it cool down, drain it to remove any remaining rust sludge, and refill again with the proper green coolant mix, which should be 50/50 to 60/40 depending on your area.