So it just got cold here, really cold (when I'm driving to school in the morning it's between 5-10 sometimes all the way down in low negatives F). My car is in a garage at night so it's not super freezing in the morning but not exactly warm either and the heater does not work well. Air does come out of all the vents (including the middle 3) but its not warm air even when the coolant is completely warmed up. I replaced my thermostat recently so that's not the problem, does this sound like a bad heater core?

And when it is that cold outside my car has a rough idle (my car normally idles at about 950 but when it's cold will sink to 800 and hake then go back up to 950 when I'm at a light or something) idk if its really a problem or if I should just give it a little gas (1,100 rpm) or something at idle

 

Sounds like your tearing your dash out in the summer.

 

So it is the heating core?

 

Most likely, if your blower motor is blowing air and the car temp is solid its your heater core.

 

Could also be the blend door isn't closing all the way so that you're getting too much fresh air going paste the heater core. I've also heard of the foam "gasket" around the blend door letting too much air past.

Either way you're still pulling the air box out, and you might as well do a heater core once you do all that work. But take a good look at the condition of the blend door too.